diff --git a/AK/BumpAllocator.h b/AK/BumpAllocator.h
index e505e5381dd..cd2f4f4d95e 100644
--- a/AK/BumpAllocator.h
+++ b/AK/BumpAllocator.h
@@ -103,7 +103,7 @@ protected:
         void* new_chunk = (void*)s_unused_allocation_cache.exchange(0);
         if (!new_chunk) {
             if constexpr (use_mmap) {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
                 new_chunk = serenity_mmap(nullptr, m_chunk_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_RANDOMIZED | MAP_PRIVATE, 0, 0, m_chunk_size, "BumpAllocator Chunk");
 #else
                 new_chunk = mmap(nullptr, m_chunk_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
diff --git a/AK/Error.h b/AK/Error.h
index 0ab143d6e7f..56265bf3c76 100644
--- a/AK/Error.h
+++ b/AK/Error.h
@@ -10,7 +10,7 @@
 #include <AK/Try.h>
 #include <AK/Variant.h>
 
-#if defined(__serenity__) && defined(KERNEL)
+#if defined(AK_OS_SERENITY) && defined(KERNEL)
 #    include <LibC/errno_codes.h>
 #else
 #    include <errno.h>
@@ -86,7 +86,7 @@ public:
     {
     }
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     ErrorOr(ErrnoCode code)
         : m_value_or_error(Error::from_errno(code))
     {
diff --git a/AK/Format.cpp b/AK/Format.cpp
index ee68a18f519..e88f516c098 100644
--- a/AK/Format.cpp
+++ b/AK/Format.cpp
@@ -11,7 +11,7 @@
 #include <AK/StringBuilder.h>
 #include <AK/kstdio.h>
 
-#if defined(__serenity__) && !defined(KERNEL)
+#if defined(AK_OS_SERENITY) && !defined(KERNEL)
 #    include <serenity.h>
 #endif
 
@@ -877,7 +877,7 @@ void vdbgln(StringView fmtstr, TypeErasedFormatParams& params)
 
     StringBuilder builder;
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    ifdef KERNEL
     if (Kernel::Processor::is_initialized()) {
         struct timespec ts = {};
@@ -914,7 +914,7 @@ void vdbgln(StringView fmtstr, TypeErasedFormatParams& params)
 
     auto const string = builder.string_view();
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    ifdef KERNEL
     if (!Kernel::Processor::is_initialized()) {
         kernelearlyputstr(string.characters_without_null_termination(), string.length());
@@ -930,7 +930,7 @@ void vdmesgln(StringView fmtstr, TypeErasedFormatParams& params)
 {
     StringBuilder builder;
 
-#    ifdef __serenity__
+#    ifdef AK_OS_SERENITY
     struct timespec ts = {};
 
 #        if !ARCH(AARCH64)
@@ -959,7 +959,7 @@ void v_critical_dmesgln(StringView fmtstr, TypeErasedFormatParams& params)
     // at OOM conditions.
 
     StringBuilder builder;
-#    ifdef __serenity__
+#    ifdef AK_OS_SERENITY
     if (Kernel::Processor::is_initialized() && Kernel::Thread::current()) {
         auto& thread = *Kernel::Thread::current();
         builder.appendff("[{}({}:{})]: ", thread.process().name(), thread.pid().value(), thread.tid().value());
diff --git a/AK/Format.h b/AK/Format.h
index d89699bde5e..dcba9d81dd4 100644
--- a/AK/Format.h
+++ b/AK/Format.h
@@ -671,7 +671,7 @@ template<>
 struct Formatter<Error> : Formatter<FormatString> {
     ErrorOr<void> format(FormatBuilder& builder, Error const& error)
     {
-#if defined(__serenity__) && defined(KERNEL)
+#if defined(AK_OS_SERENITY) && defined(KERNEL)
         if (error.is_errno())
             return Formatter<FormatString>::format(builder, "Error(errno={})"sv, error.code());
         return Formatter<FormatString>::format(builder, "Error({})"sv, error.string_literal());
diff --git a/AK/NoAllocationGuard.h b/AK/NoAllocationGuard.h
index d48b5af6e08..544057809b1 100644
--- a/AK/NoAllocationGuard.h
+++ b/AK/NoAllocationGuard.h
@@ -39,7 +39,7 @@ private:
     {
 #if defined(KERNEL)
         return Processor::current_thread()->get_allocation_enabled();
-#elif defined(__serenity__)
+#elif defined(AK_OS_SERENITY)
         // This extern thread-local lives in our LibC, which doesn't exist on other systems.
         return s_allocation_enabled;
 #else
@@ -51,7 +51,7 @@ private:
     {
 #if defined(KERNEL)
         Processor::current_thread()->set_allocation_enabled(value);
-#elif defined(__serenity__)
+#elif defined(AK_OS_SERENITY)
         s_allocation_enabled = value;
 #else
         (void)value;
diff --git a/AK/Platform.h b/AK/Platform.h
index e6bf438626e..7f6003bff54 100644
--- a/AK/Platform.h
+++ b/AK/Platform.h
@@ -148,7 +148,7 @@
 #    define ASAN_UNPOISON_MEMORY_REGION(addr, size)
 #endif
 
-#ifndef __serenity__
+#ifndef AK_OS_SERENITY
 // On macOS (at least Mojave), Apple's version of this header is not wrapped
 // in extern "C".
 #    ifdef AK_OS_MACOS
diff --git a/AK/PrintfImplementation.h b/AK/PrintfImplementation.h
index 03a00f58f96..5d590f1d5c6 100644
--- a/AK/PrintfImplementation.h
+++ b/AK/PrintfImplementation.h
@@ -16,7 +16,7 @@
 #    include <math.h>
 #endif
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 extern "C" size_t strlen(char const*);
 #else
 #    include <string.h>
diff --git a/AK/Random.h b/AK/Random.h
index 4767c93e5a2..65466680c69 100644
--- a/AK/Random.h
+++ b/AK/Random.h
@@ -9,7 +9,7 @@
 #include <AK/Platform.h>
 #include <AK/Types.h>
 
-#if defined(__serenity__) || defined(AK_OS_ANDROID)
+#if defined(AK_OS_SERENITY) || defined(AK_OS_ANDROID)
 #    include <stdlib.h>
 #endif
 
@@ -30,7 +30,7 @@ namespace AK {
 
 inline void fill_with_random([[maybe_unused]] void* buffer, [[maybe_unused]] size_t length)
 {
-#if defined(__serenity__) || defined(AK_OS_ANDROID)
+#if defined(AK_OS_SERENITY) || defined(AK_OS_ANDROID)
     arc4random_buf(buffer, length);
 #elif defined(OSS_FUZZ)
 #elif defined(__unix__) or defined(AK_OS_MACOS)
diff --git a/AK/Singleton.h b/AK/Singleton.h
index c74b3324697..3365309d283 100644
--- a/AK/Singleton.h
+++ b/AK/Singleton.h
@@ -23,7 +23,7 @@ extern "C" __declspec(dllimport) void __stdcall Sleep(unsigned long);
 #    include <sched.h>
 #endif
 
-#ifndef __serenity__
+#ifndef AK_OS_SERENITY
 #    include <new>
 #endif
 
diff --git a/AK/StackInfo.cpp b/AK/StackInfo.cpp
index 4f53947881a..95384f1e08c 100644
--- a/AK/StackInfo.cpp
+++ b/AK/StackInfo.cpp
@@ -10,7 +10,7 @@
 #include <stdio.h>
 #include <string.h>
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    include <serenity.h>
 #elif defined(__linux__) or defined(AK_OS_MACOS)
 #    include <pthread.h>
@@ -23,7 +23,7 @@ namespace AK {
 
 StackInfo::StackInfo()
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     if (get_stack_bounds(&m_base, &m_size) < 0) {
         perror("get_stack_bounds");
         VERIFY_NOT_REACHED();
diff --git a/AK/Types.h b/AK/Types.h
index 06e35d4d9cb..7b16d75362e 100644
--- a/AK/Types.h
+++ b/AK/Types.h
@@ -19,7 +19,7 @@ using i32 = __INT32_TYPE__;
 using i16 = __INT16_TYPE__;
 using i8 = __INT8_TYPE__;
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 
 using size_t = __SIZE_TYPE__;
 using ssize_t = MakeSigned<size_t>;
diff --git a/AK/kmalloc.cpp b/AK/kmalloc.cpp
index cba00d36078..0ce83fddab1 100644
--- a/AK/kmalloc.cpp
+++ b/AK/kmalloc.cpp
@@ -5,10 +5,11 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#if defined(__serenity__) && !defined(KERNEL)
+#include <AK/kmalloc.h>
+
+#if defined(AK_OS_SERENITY) && !defined(KERNEL)
 
 #    include <AK/Assertions.h>
-#    include <AK/kmalloc.h>
 
 // However deceptively simple these functions look, they must not be inlined.
 // Memory allocated in one translation unit has to be deallocatable in another
diff --git a/AK/kmalloc.h b/AK/kmalloc.h
index 481e2e8ce8c..a6188bff327 100644
--- a/AK/kmalloc.h
+++ b/AK/kmalloc.h
@@ -8,6 +8,7 @@
 #pragma once
 
 #include <AK/Checked.h>
+#include <AK/Platform.h>
 
 #if defined(KERNEL)
 #    include <Kernel/Heap/kmalloc.h>
@@ -25,7 +26,7 @@ inline void kfree_sized(void* ptr, size_t)
 }
 #endif
 
-#ifndef __serenity__
+#ifndef AK_OS_SERENITY
 #    include <AK/Types.h>
 
 #    ifndef AK_OS_MACOS
diff --git a/AK/kstdio.h b/AK/kstdio.h
index 2ef6b341aa0..2e2a05ac217 100644
--- a/AK/kstdio.h
+++ b/AK/kstdio.h
@@ -6,7 +6,9 @@
 
 #pragma once
 
-#ifdef __serenity__
+#include <AK/Platform.h>
+
+#ifdef AK_OS_SERENITY
 #    ifdef KERNEL
 #        include <Kernel/kstdio.h>
 #    else
diff --git a/Kernel/API/Syscall.h b/Kernel/API/Syscall.h
index 8e81e9dc3a7..be5cfbd78b3 100644
--- a/Kernel/API/Syscall.h
+++ b/Kernel/API/Syscall.h
@@ -204,7 +204,7 @@ enum Function {
         __Count
 };
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 struct StringArgument {
     char const* characters;
     size_t length { 0 };
diff --git a/Tests/LibCompress/TestBrotli.cpp b/Tests/LibCompress/TestBrotli.cpp
index 5c4e9788123..e5e14ebf032 100644
--- a/Tests/LibCompress/TestBrotli.cpp
+++ b/Tests/LibCompress/TestBrotli.cpp
@@ -12,7 +12,7 @@
 static void run_test(StringView const file_name)
 {
     // This makes sure that the tests will run both on target and in Lagom.
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     String path = String::formatted("/usr/Tests/LibCompress/brotli-test-files/{}", file_name);
 #else
     String path = String::formatted("brotli-test-files/{}", file_name);
@@ -83,7 +83,7 @@ TEST_CASE(brotli_single_z)
 TEST_CASE(brotli_decompress_zero_one_bin)
 {
     // This makes sure that the tests will run both on target and in Lagom.
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     String path = "/usr/Tests/LibCompress/brotli-test-files/zero-one.bin";
 #else
     String path = "brotli-test-files/zero-one.bin";
diff --git a/Tests/LibGL/TestRender.cpp b/Tests/LibGL/TestRender.cpp
index 9b01dfac989..240f8f23e1a 100644
--- a/Tests/LibGL/TestRender.cpp
+++ b/Tests/LibGL/TestRender.cpp
@@ -14,7 +14,7 @@
 #include <LibGfx/QOIWriter.h>
 #include <LibTest/TestCase.h>
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    define REFERENCE_IMAGE_DIR "/usr/Tests/LibGL/reference-images"
 #else
 #    define REFERENCE_IMAGE_DIR "reference-images"
diff --git a/Tests/LibJS/test262-runner.cpp b/Tests/LibJS/test262-runner.cpp
index f88dcc2fd3d..933a722fbc5 100644
--- a/Tests/LibJS/test262-runner.cpp
+++ b/Tests/LibJS/test262-runner.cpp
@@ -544,7 +544,7 @@ static bool g_in_assert = false;
     exit(12);
 }
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 void __assertion_failed(char const* assertion)
 {
     handle_failed_assert(assertion);
diff --git a/Tests/Spreadsheet/test-spreadsheet.cpp b/Tests/Spreadsheet/test-spreadsheet.cpp
index 838164ae5eb..0d87a1d628c 100644
--- a/Tests/Spreadsheet/test-spreadsheet.cpp
+++ b/Tests/Spreadsheet/test-spreadsheet.cpp
@@ -9,7 +9,7 @@
 
 TEST_ROOT("Userland/Applications/Spreadsheet/Tests");
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 static constexpr auto s_spreadsheet_runtime_path = "/res/js/Spreadsheet/runtime.js"sv;
 #else
 static constexpr auto s_spreadsheet_runtime_path = "../../../../Base/res/js/Spreadsheet/runtime.js"sv;
@@ -32,7 +32,7 @@ TESTJS_RUN_FILE_FUNCTION(String const&, JS::Interpreter& interpreter, JS::Execut
         interpreter.vm().pop_execution_context();
     };
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     run_file(s_spreadsheet_runtime_path);
 #else
     run_file(LexicalPath::join(Test::JS::g_test_root, s_spreadsheet_runtime_path).string());
diff --git a/Userland/Libraries/LibCore/Command.cpp b/Userland/Libraries/LibCore/Command.cpp
index ac38243432e..bc2f73378d1 100644
--- a/Userland/Libraries/LibCore/Command.cpp
+++ b/Userland/Libraries/LibCore/Command.cpp
@@ -16,7 +16,7 @@
 namespace Core {
 
 // Only supported in serenity mode because we use `posix_spawn_file_actions_addchdir`
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 
 ErrorOr<CommandResult> command(String const& command_string, Optional<LexicalPath> chdir)
 {
diff --git a/Userland/Libraries/LibCore/EventLoop.cpp b/Userland/Libraries/LibCore/EventLoop.cpp
index 60e5c96aeb4..c72dd21cf29 100644
--- a/Userland/Libraries/LibCore/EventLoop.cpp
+++ b/Userland/Libraries/LibCore/EventLoop.cpp
@@ -36,7 +36,7 @@
 #include <time.h>
 #include <unistd.h>
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    include <LibCore/Account.h>
 
 extern bool s_global_initializers_ran;
@@ -160,7 +160,7 @@ private:
             return allocator->allocate();
         }))
     {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         m_socket->on_ready_to_read = [this] {
             u32 length;
             auto maybe_bytes_read = m_socket->read({ (u8*)&length, sizeof(length) });
@@ -236,7 +236,7 @@ public:
             JsonObject response;
             response.set("type", type);
             response.set("pid", getpid());
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
             char buffer[1024];
             if (get_process_name(buffer, sizeof(buffer)) >= 0) {
                 response.set("process_name", buffer);
@@ -312,7 +312,7 @@ EventLoop::EventLoop([[maybe_unused]] MakeInspectable make_inspectable)
     : m_wake_pipe_fds(&s_wake_pipe_fds)
     , m_private(make<Private>())
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     if (!s_global_initializers_ran) {
         // NOTE: Trying to have an event loop as a global variable will lead to initialization-order fiascos,
         //       as the event loop constructor accesses and/or sets other global variables.
@@ -334,7 +334,7 @@ EventLoop::EventLoop([[maybe_unused]] MakeInspectable make_inspectable)
         s_pid = getpid();
         s_event_loop_stack->append(*this);
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         if (getuid() != 0) {
             if (getenv("MAKE_INSPECTABLE") == "1"sv)
                 make_inspectable = Core::EventLoop::MakeInspectable::Yes;
@@ -361,7 +361,7 @@ EventLoop::~EventLoop()
 
 bool connect_to_inspector_server()
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     auto maybe_path = SessionManagement::parse_path_with_sid("/tmp/session/%sid/portal/inspectables"sv);
     if (maybe_path.is_error()) {
         dbgln("connect_to_inspector_server: {}", maybe_path.error());
diff --git a/Userland/Libraries/LibCore/File.cpp b/Userland/Libraries/LibCore/File.cpp
index c3a83a5c854..94014771d9b 100644
--- a/Userland/Libraries/LibCore/File.cpp
+++ b/Userland/Libraries/LibCore/File.cpp
@@ -4,9 +4,6 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
-#ifdef __serenity__
-#    include <serenity.h>
-#endif
 #include <AK/LexicalPath.h>
 #include <AK/ScopeGuard.h>
 #include <LibCore/DirIterator.h>
@@ -20,6 +17,10 @@
 #include <unistd.h>
 #include <utime.h>
 
+#ifdef AK_OS_SERENITY
+#    include <serenity.h>
+#endif
+
 // On Linux distros that use glibc `basename` is defined as a macro that expands to `__xpg_basename`, so we undefine it
 #if defined(__linux__) && defined(basename)
 #    undef basename
@@ -241,7 +242,7 @@ String File::absolute_path(String const& path)
     return LexicalPath::canonicalized_path(full_path.string());
 }
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 
 ErrorOr<String> File::read_link(String const& link_path)
 {
diff --git a/Userland/Libraries/LibCore/FileWatcher.cpp b/Userland/Libraries/LibCore/FileWatcher.cpp
index 73ae7ce64eb..57ebfb33840 100644
--- a/Userland/Libraries/LibCore/FileWatcher.cpp
+++ b/Userland/Libraries/LibCore/FileWatcher.cpp
@@ -21,7 +21,7 @@
 namespace Core {
 
 // Only supported in serenity mode because we use InodeWatcher syscalls
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 
 static Optional<FileWatcherEvent> get_event_from_fd(int fd, HashMap<unsigned, String> const& wd_to_path)
 {
diff --git a/Userland/Libraries/LibCore/Process.cpp b/Userland/Libraries/LibCore/Process.cpp
index a7c5747dc66..623a71a780c 100644
--- a/Userland/Libraries/LibCore/Process.cpp
+++ b/Userland/Libraries/LibCore/Process.cpp
@@ -12,7 +12,7 @@
 #include <errno.h>
 #include <spawn.h>
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    include <serenity.h>
 #endif
 
@@ -46,7 +46,7 @@ struct ArgvList {
     ErrorOr<pid_t> spawn()
     {
         auto pid = TRY(System::posix_spawn(m_path.view(), nullptr, nullptr, const_cast<char**>(get().data()), environ));
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         TRY(System::disown(pid));
 #endif
         return pid;
diff --git a/Userland/Libraries/LibCore/Stream.cpp b/Userland/Libraries/LibCore/Stream.cpp
index bab2474af58..6d8cd80fee9 100644
--- a/Userland/Libraries/LibCore/Stream.cpp
+++ b/Userland/Libraries/LibCore/Stream.cpp
@@ -14,7 +14,7 @@
 #include <sys/socket.h>
 #include <sys/types.h>
 #include <unistd.h>
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    include <serenity.h>
 #endif
 #ifdef __FreeBSD__
diff --git a/Userland/Libraries/LibCore/System.cpp b/Userland/Libraries/LibCore/System.cpp
index c156d14daa4..6359b34d5d5 100644
--- a/Userland/Libraries/LibCore/System.cpp
+++ b/Userland/Libraries/LibCore/System.cpp
@@ -25,7 +25,7 @@
 #include <termios.h>
 #include <unistd.h>
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    include <LibCore/Account.h>
 #    include <LibSystem/syscall.h>
 #    include <serenity.h>
@@ -61,7 +61,7 @@ namespace Core::System {
 #    endif
 #endif
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 
 ErrorOr<void> beep()
 {
@@ -256,7 +256,7 @@ ErrorOr<int> fcntl(int fd, int command, ...)
 
 ErrorOr<void*> mmap(void* address, size_t size, int protection, int flags, int fd, off_t offset, [[maybe_unused]] size_t alignment, [[maybe_unused]] StringView name)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_mmap_params params { address, size, alignment, protection, flags, fd, offset, { name.characters_without_null_termination(), name.length() } };
     ptrdiff_t rc = syscall(SC_mmap, &params);
     if (rc < 0 && rc > -EMAXERRNO)
@@ -282,7 +282,7 @@ ErrorOr<void> munmap(void* address, size_t size)
 ErrorOr<int> anon_create([[maybe_unused]] size_t size, [[maybe_unused]] int options)
 {
     int fd = -1;
-#if defined(__serenity__)
+#if defined(AK_OS_SERENITY)
     fd = ::anon_create(round_up_to_power_of_two(size, PAGE_SIZE), options);
 #elif defined(__linux__) || defined(__FreeBSD__)
     // FIXME: Support more options on Linux.
@@ -337,7 +337,7 @@ ErrorOr<int> openat(int fd, StringView path, int options, mode_t mode)
 {
     if (!path.characters_without_null_termination())
         return Error::from_syscall("open"sv, -EFAULT);
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_open_params params { fd, { path.characters_without_null_termination(), path.length() }, options, mode };
     int rc = syscall(SC_open, &params);
     HANDLE_SYSCALL_RETURN_VALUE("open", rc, rc);
@@ -371,7 +371,7 @@ ErrorOr<struct stat> stat(StringView path)
         return Error::from_syscall("stat"sv, -EFAULT);
 
     struct stat st = {};
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_stat_params params { { path.characters_without_null_termination(), path.length() }, &st, AT_FDCWD, true };
     int rc = syscall(SC_stat, &params);
     HANDLE_SYSCALL_RETURN_VALUE("stat", rc, st);
@@ -389,7 +389,7 @@ ErrorOr<struct stat> lstat(StringView path)
         return Error::from_syscall("lstat"sv, -EFAULT);
 
     struct stat st = {};
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_stat_params params { { path.characters_without_null_termination(), path.length() }, &st, AT_FDCWD, false };
     int rc = syscall(SC_stat, &params);
     HANDLE_SYSCALL_RETURN_VALUE("lstat", rc, st);
@@ -522,7 +522,7 @@ ErrorOr<void> chmod(StringView pathname, mode_t mode)
     if (!pathname.characters_without_null_termination())
         return Error::from_syscall("chmod"sv, -EFAULT);
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_chmod_params params {
         AT_FDCWD,
         { pathname.characters_without_null_termination(), pathname.length() },
@@ -558,7 +558,7 @@ ErrorOr<void> lchown(StringView pathname, uid_t uid, gid_t gid)
     if (!pathname.characters_without_null_termination())
         return Error::from_syscall("chown"sv, -EFAULT);
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_chown_params params = { { pathname.characters_without_null_termination(), pathname.length() }, uid, gid, AT_FDCWD, false };
     int rc = syscall(SC_chown, &params);
     HANDLE_SYSCALL_RETURN_VALUE("chown", rc, {});
@@ -575,7 +575,7 @@ ErrorOr<void> chown(StringView pathname, uid_t uid, gid_t gid)
     if (!pathname.characters_without_null_termination())
         return Error::from_syscall("chown"sv, -EFAULT);
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_chown_params params = { { pathname.characters_without_null_termination(), pathname.length() }, uid, gid, AT_FDCWD, true };
     int rc = syscall(SC_chown, &params);
     HANDLE_SYSCALL_RETURN_VALUE("chown", rc, {});
@@ -649,7 +649,7 @@ ErrorOr<Optional<struct group>> getgrnam(StringView name)
 
 ErrorOr<void> clock_settime(clockid_t clock_id, struct timespec* ts)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     int rc = syscall(SC_clock_settime, clock_id, ts);
     HANDLE_SYSCALL_RETURN_VALUE("clocksettime", rc, {});
 #else
@@ -777,7 +777,7 @@ ErrorOr<bool> isatty(int fd)
 
 ErrorOr<void> link(StringView old_path, StringView new_path)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_link_params params {
         .old_path = { old_path.characters_without_null_termination(), old_path.length() },
         .new_path = { new_path.characters_without_null_termination(), new_path.length() },
@@ -795,7 +795,7 @@ ErrorOr<void> link(StringView old_path, StringView new_path)
 
 ErrorOr<void> symlink(StringView target, StringView link_path)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_symlink_params params {
         .target = { target.characters_without_null_termination(), target.length() },
         .linkpath = { link_path.characters_without_null_termination(), link_path.length() },
@@ -815,7 +815,7 @@ ErrorOr<void> mkdir(StringView path, mode_t mode)
 {
     if (path.is_null())
         return Error::from_errno(EFAULT);
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     int rc = syscall(SC_mkdir, path.characters_without_null_termination(), path.length(), mode);
     HANDLE_SYSCALL_RETURN_VALUE("mkdir", rc, {});
 #else
@@ -830,7 +830,7 @@ ErrorOr<void> chdir(StringView path)
 {
     if (path.is_null())
         return Error::from_errno(EFAULT);
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     int rc = syscall(SC_chdir, path.characters_without_null_termination(), path.length());
     HANDLE_SYSCALL_RETURN_VALUE("chdir", rc, {});
 #else
@@ -845,7 +845,7 @@ ErrorOr<void> rmdir(StringView path)
 {
     if (path.is_null())
         return Error::from_errno(EFAULT);
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     int rc = syscall(SC_rmdir, path.characters_without_null_termination(), path.length());
     HANDLE_SYSCALL_RETURN_VALUE("rmdir", rc, {});
 #else
@@ -877,7 +877,7 @@ ErrorOr<void> rename(StringView old_path, StringView new_path)
     if (old_path.is_null() || new_path.is_null())
         return Error::from_errno(EFAULT);
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_rename_params params {
         .old_path = { old_path.characters_without_null_termination(), old_path.length() },
         .new_path = { new_path.characters_without_null_termination(), new_path.length() },
@@ -898,7 +898,7 @@ ErrorOr<void> unlink(StringView path)
     if (path.is_null())
         return Error::from_errno(EFAULT);
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     int rc = syscall(SC_unlink, AT_FDCWD, path.characters_without_null_termination(), path.length(), 0);
     HANDLE_SYSCALL_RETURN_VALUE("unlink", rc, {});
 #else
@@ -917,7 +917,7 @@ ErrorOr<void> utime(StringView path, Optional<struct utimbuf> maybe_buf)
     struct utimbuf* buf = nullptr;
     if (maybe_buf.has_value())
         buf = &maybe_buf.value();
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     int rc = syscall(SC_utime, path.characters_without_null_termination(), path.length(), buf);
     HANDLE_SYSCALL_RETURN_VALUE("utime", rc, {});
 #else
@@ -931,7 +931,7 @@ ErrorOr<void> utime(StringView path, Optional<struct utimbuf> maybe_buf)
 ErrorOr<struct utsname> uname()
 {
     utsname uts;
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     int rc = syscall(SC_uname, &uts);
     HANDLE_SYSCALL_RETURN_VALUE("uname", rc, uts);
 #else
@@ -944,7 +944,7 @@ ErrorOr<struct utsname> uname()
 #ifndef AK_OS_ANDROID
 ErrorOr<void> adjtime(const struct timeval* delta, struct timeval* old_delta)
 {
-#    ifdef __serenity__
+#    ifdef AK_OS_SERENITY
     int rc = syscall(SC_adjtime, delta, old_delta);
     HANDLE_SYSCALL_RETURN_VALUE("adjtime", rc, {});
 #    else
@@ -957,7 +957,7 @@ ErrorOr<void> adjtime(const struct timeval* delta, struct timeval* old_delta)
 
 ErrorOr<void> exec(StringView filename, Span<StringView> arguments, SearchInPath search_in_path, Optional<Span<StringView>> environment)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_execve_params params;
 
     auto argument_strings = TRY(FixedArray<Syscall::StringArgument>::try_create(arguments.size()));
@@ -1231,7 +1231,7 @@ ErrorOr<void> mknod(StringView pathname, mode_t mode, dev_t dev)
     if (pathname.is_null())
         return Error::from_syscall("mknod"sv, -EFAULT);
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     Syscall::SC_mknod_params params { { pathname.characters_without_null_termination(), pathname.length() }, mode, dev };
     int rc = syscall(SC_mknod, &params);
     HANDLE_SYSCALL_RETURN_VALUE("mknod", rc, {});
@@ -1250,7 +1250,7 @@ ErrorOr<void> mkfifo(StringView pathname, mode_t mode)
 
 ErrorOr<void> setenv(StringView name, StringView value, bool overwrite)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     auto const rc = ::serenity_setenv(name.characters_without_null_termination(), name.length(), value.characters_without_null_termination(), value.length(), overwrite);
 #else
     String name_string = name;
@@ -1291,7 +1291,7 @@ ErrorOr<void> access(StringView pathname, int mode)
     if (pathname.is_null())
         return Error::from_syscall("access"sv, -EFAULT);
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     int rc = ::syscall(Syscall::SC_access, pathname.characters_without_null_termination(), pathname.length(), mode);
     HANDLE_SYSCALL_RETURN_VALUE("access", rc, {});
 #else
diff --git a/Userland/Libraries/LibCore/System.h b/Userland/Libraries/LibCore/System.h
index fd9a4d7603f..36badbf244e 100644
--- a/Userland/Libraries/LibCore/System.h
+++ b/Userland/Libraries/LibCore/System.h
@@ -32,7 +32,7 @@
 
 namespace Core::System {
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 ErrorOr<void> beep();
 ErrorOr<void> pledge(StringView promises, StringView execpromises = {});
 ErrorOr<void> unveil(StringView path, StringView permissions);
diff --git a/Userland/Libraries/LibELF/DynamicLoader.cpp b/Userland/Libraries/LibELF/DynamicLoader.cpp
index 6d28491d5ab..1e7cd74032a 100644
--- a/Userland/Libraries/LibELF/DynamicLoader.cpp
+++ b/Userland/Libraries/LibELF/DynamicLoader.cpp
@@ -26,7 +26,7 @@
 #include <sys/stat.h>
 #include <unistd.h>
 
-#ifndef __serenity__
+#ifndef AK_OS_SERENITY
 static void* mmap_with_name(void* addr, size_t length, int prot, int flags, int fd, off_t offset, char const*)
 {
     return mmap(addr, length, prot, flags, fd, offset);
@@ -239,7 +239,7 @@ Result<NonnullRefPtr<DynamicObject>, DlErrorMessage> DynamicLoader::load_stage_3
             return DlErrorMessage { String::formatted("mprotect .relro: PROT_READ: {}", strerror(errno)) };
         }
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         if (set_mmap_name(m_relro_segment_address.as_ptr(), m_relro_segment_size, String::formatted("{}: .relro", m_filepath).characters()) < 0) {
             return DlErrorMessage { String::formatted("set_mmap_name .relro: {}", strerror(errno)) };
         }
diff --git a/Userland/Libraries/LibGPU/Driver.cpp b/Userland/Libraries/LibGPU/Driver.cpp
index 7bdcad4a96f..e2d2d973a03 100644
--- a/Userland/Libraries/LibGPU/Driver.cpp
+++ b/Userland/Libraries/LibGPU/Driver.cpp
@@ -15,9 +15,9 @@ namespace GPU {
 // FIXME: Think of a better way to configure these paths. Maybe use ConfigServer?
 static HashMap<String, String> const s_driver_path_map
 {
-#if defined(__serenity__)
+#if defined(AK_OS_SERENITY)
     { "softgpu", "libsoftgpu.so.serenity" },
-#elif defined(__APPLE__)
+#elif defined(AK_OS_MACOS)
     { "softgpu", "liblagom-softgpu.dylib" },
 #else
     { "softgpu", "liblagom-softgpu.so.0" },
diff --git a/Userland/Libraries/LibGfx/Bitmap.cpp b/Userland/Libraries/LibGfx/Bitmap.cpp
index 3a6f1cb9412..bb6a0e828e3 100644
--- a/Userland/Libraries/LibGfx/Bitmap.cpp
+++ b/Userland/Libraries/LibGfx/Bitmap.cpp
@@ -498,7 +498,7 @@ Bitmap::~Bitmap()
 void Bitmap::set_mmap_name([[maybe_unused]] String const& name)
 {
     VERIFY(m_needs_munmap);
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     ::set_mmap_name(m_data, size_in_bytes(), name.characters());
 #endif
 }
@@ -516,7 +516,7 @@ void Bitmap::set_volatile()
 {
     if (m_volatile)
         return;
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     int rc = madvise(m_data, size_in_bytes(), MADV_SET_VOLATILE);
     if (rc < 0) {
         perror("madvise(MADV_SET_VOLATILE)");
@@ -533,7 +533,7 @@ void Bitmap::set_volatile()
         return true;
     }
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     int rc = madvise(m_data, size_in_bytes(), MADV_SET_NONVOLATILE);
     if (rc < 0) {
         if (errno == ENOMEM) {
@@ -566,7 +566,7 @@ ErrorOr<BackingStore> Bitmap::allocate_backing_store(BitmapFormat format, IntSiz
     auto const data_size_in_bytes = size_in_bytes(pitch, size.height() * scale_factor);
 
     int map_flags = MAP_ANONYMOUS | MAP_PRIVATE;
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     map_flags |= MAP_PURGEABLE;
     void* data = mmap_with_name(nullptr, data_size_in_bytes, PROT_READ | PROT_WRITE, map_flags, 0, 0, String::formatted("GraphicsBitmap [{}]", size).characters());
 #else
diff --git a/Userland/Libraries/LibJS/Console.cpp b/Userland/Libraries/LibJS/Console.cpp
index b8bdd161e7f..165723e7c66 100644
--- a/Userland/Libraries/LibJS/Console.cpp
+++ b/Userland/Libraries/LibJS/Console.cpp
@@ -429,7 +429,7 @@ MarkedVector<Value> Console::vm_arguments()
 
 void Console::output_debug_message([[maybe_unused]] LogLevel log_level, [[maybe_unused]] String output) const
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     switch (log_level) {
     case Console::LogLevel::Debug:
         dbgln("\033[32;1m(js debug)\033[0m {}", output);
diff --git a/Userland/Libraries/LibJS/Heap/BlockAllocator.cpp b/Userland/Libraries/LibJS/Heap/BlockAllocator.cpp
index c3ab61237a4..a351bfa45c0 100644
--- a/Userland/Libraries/LibJS/Heap/BlockAllocator.cpp
+++ b/Userland/Libraries/LibJS/Heap/BlockAllocator.cpp
@@ -21,7 +21,7 @@ BlockAllocator::~BlockAllocator()
 {
     for (auto* block : m_blocks) {
         ASAN_UNPOISON_MEMORY_REGION(block, HeapBlock::block_size);
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         if (munmap(block, HeapBlock::block_size) < 0) {
             perror("munmap");
             VERIFY_NOT_REACHED();
@@ -39,7 +39,7 @@ void* BlockAllocator::allocate_block([[maybe_unused]] char const* name)
         size_t random_index = get_random_uniform(m_blocks.size());
         auto* block = m_blocks.unstable_take(random_index);
         ASAN_UNPOISON_MEMORY_REGION(block, HeapBlock::block_size);
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         if (set_mmap_name(block, HeapBlock::block_size, name) < 0) {
             perror("set_mmap_name");
             VERIFY_NOT_REACHED();
@@ -48,7 +48,7 @@ void* BlockAllocator::allocate_block([[maybe_unused]] char const* name)
         return block;
     }
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     auto* block = (HeapBlock*)serenity_mmap(nullptr, HeapBlock::block_size, PROT_READ | PROT_WRITE, MAP_ANONYMOUS | MAP_RANDOMIZED | MAP_PRIVATE, 0, 0, HeapBlock::block_size, name);
     VERIFY(block != MAP_FAILED);
 #else
@@ -62,7 +62,7 @@ void BlockAllocator::deallocate_block(void* block)
 {
     VERIFY(block);
     if (m_blocks.size() >= max_cached_blocks) {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         if (munmap(block, HeapBlock::block_size) < 0) {
             perror("munmap");
             VERIFY_NOT_REACHED();
diff --git a/Userland/Libraries/LibJS/Heap/Heap.cpp b/Userland/Libraries/LibJS/Heap/Heap.cpp
index a6eee105a6e..a9860b94736 100644
--- a/Userland/Libraries/LibJS/Heap/Heap.cpp
+++ b/Userland/Libraries/LibJS/Heap/Heap.cpp
@@ -20,13 +20,13 @@
 #include <LibJS/SafeFunction.h>
 #include <setjmp.h>
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    include <serenity.h>
 #endif
 
 namespace JS {
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 static int gc_perf_string_id;
 #endif
 
@@ -36,7 +36,7 @@ static __thread HashMap<FlatPtr*, size_t>* s_custom_ranges_for_conservative_scan
 Heap::Heap(VM& vm)
     : m_vm(vm)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     auto gc_signpost_string = "Garbage collection"sv;
     gc_perf_string_id = perf_register_string(gc_signpost_string.characters_without_null_termination(), gc_signpost_string.length());
 #endif
@@ -91,7 +91,7 @@ void Heap::collect_garbage(CollectionType collection_type, bool print_report)
     VERIFY(!m_collecting_garbage);
     TemporaryChange change(m_collecting_garbage, true);
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     static size_t global_gc_counter = 0;
     perf_event(PERF_EVENT_SIGNPOST, gc_perf_string_id, global_gc_counter++);
 #endif
diff --git a/Userland/Libraries/LibJS/Heap/HeapBlock.cpp b/Userland/Libraries/LibJS/Heap/HeapBlock.cpp
index fd997857d22..6fe8c6e4e40 100644
--- a/Userland/Libraries/LibJS/Heap/HeapBlock.cpp
+++ b/Userland/Libraries/LibJS/Heap/HeapBlock.cpp
@@ -20,7 +20,7 @@ namespace JS {
 
 NonnullOwnPtr<HeapBlock> HeapBlock::create_with_cell_size(Heap& heap, size_t cell_size)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     char name[64];
     snprintf(name, sizeof(name), "LibJS: HeapBlock(%zu)", cell_size);
 #else
diff --git a/Userland/Libraries/LibJS/Runtime/GlobalObject.cpp b/Userland/Libraries/LibJS/Runtime/GlobalObject.cpp
index 003add64d5c..68f5b1ec532 100644
--- a/Userland/Libraries/LibJS/Runtime/GlobalObject.cpp
+++ b/Userland/Libraries/LibJS/Runtime/GlobalObject.cpp
@@ -200,7 +200,7 @@ GlobalObject::~GlobalObject() = default;
 
 JS_DEFINE_NATIVE_FUNCTION(GlobalObject::gc)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     dbgln("Forced garbage collection requested!");
 #endif
     vm.heap().collect_garbage();
diff --git a/Userland/Libraries/LibMain/Main.cpp b/Userland/Libraries/LibMain/Main.cpp
index 69f6e30fa81..6b0604c02e4 100644
--- a/Userland/Libraries/LibMain/Main.cpp
+++ b/Userland/Libraries/LibMain/Main.cpp
@@ -44,7 +44,7 @@ int main(int argc, char** argv)
     if (result.is_error()) {
         auto error = result.release_error();
         warnln("\033[31;1mRuntime error\033[0m: {}", error);
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         dbgln("\033[31;1mExiting with runtime error\033[0m: {}", error);
 #endif
         return Main::return_code_for_errors();
diff --git a/Userland/Libraries/LibRegex/RegexError.h b/Userland/Libraries/LibRegex/RegexError.h
index 9d4d5060c99..c778c2e258a 100644
--- a/Userland/Libraries/LibRegex/RegexError.h
+++ b/Userland/Libraries/LibRegex/RegexError.h
@@ -8,7 +8,7 @@
 
 #include <AK/String.h>
 #include <AK/Types.h>
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    include <bits/regex_defs.h>
 #else
 #    include <LibC/bits/regex_defs.h>
diff --git a/Userland/Libraries/LibRegex/RegexOptions.h b/Userland/Libraries/LibRegex/RegexOptions.h
index d369770c940..a292783128b 100644
--- a/Userland/Libraries/LibRegex/RegexOptions.h
+++ b/Userland/Libraries/LibRegex/RegexOptions.h
@@ -8,7 +8,7 @@
 
 #include <AK/Types.h>
 #include <stdio.h>
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    include <bits/regex_defs.h>
 #else
 #    include <LibC/bits/regex_defs.h>
diff --git a/Userland/Libraries/LibTest/JavaScriptTestRunner.h b/Userland/Libraries/LibTest/JavaScriptTestRunner.h
index bad1cdf63d4..b2d4e01fe85 100644
--- a/Userland/Libraries/LibTest/JavaScriptTestRunner.h
+++ b/Userland/Libraries/LibTest/JavaScriptTestRunner.h
@@ -37,7 +37,7 @@
 #include <sys/time.h>
 #include <unistd.h>
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    include <serenity.h>
 #endif
 
@@ -292,7 +292,7 @@ inline JSFileResult TestRunner::run_file_test(String const& test_path)
 {
     g_currently_running_test = test_path;
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     auto string_id = perf_register_string(test_path.characters(), test_path.length());
     perf_event(PERF_EVENT_SIGNPOST, string_id, 0);
 #endif
@@ -499,7 +499,7 @@ inline void TestRunner::print_file_result(JSFileResult const& file_result) const
 
     if (!file_result.logged_messages.is_empty()) {
         print_modifiers({ FG_GRAY, FG_BOLD });
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         outln("     ℹ Console output:");
 #else
         // This emoji has a second invisible byte after it. The one above does not
@@ -514,7 +514,7 @@ inline void TestRunner::print_file_result(JSFileResult const& file_result) const
         auto test_error = file_result.error.value();
 
         print_modifiers({ FG_RED });
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         outln("     ❌ The file failed to parse");
 #else
         // No invisible byte here, but the spacing still needs to be altered on the host
@@ -541,14 +541,14 @@ inline void TestRunner::print_file_result(JSFileResult const& file_result) const
             print_modifiers({ FG_GRAY, FG_BOLD });
 
             if (failed) {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
                 out("     ❌ Suite:  ");
 #else
                 // No invisible byte here, but the spacing still needs to be altered on the host
                 out("    ❌ Suite:  ");
 #endif
             } else {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
                 out("     ⚠ Suite:  ");
 #else
                 // This emoji has a second invisible byte after it. The one above does not
diff --git a/Userland/Libraries/LibTest/JavaScriptTestRunnerMain.cpp b/Userland/Libraries/LibTest/JavaScriptTestRunnerMain.cpp
index 9496615516b..fa6fc4a36f7 100644
--- a/Userland/Libraries/LibTest/JavaScriptTestRunnerMain.cpp
+++ b/Userland/Libraries/LibTest/JavaScriptTestRunnerMain.cpp
@@ -81,7 +81,7 @@ int main(int argc, char** argv)
 
     bool print_times = false;
     bool print_progress =
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         true; // Use OSC 9 to print progress
 #else
         false;
@@ -140,7 +140,7 @@ int main(int argc, char** argv)
     if (specified_test_root) {
         test_root = String { specified_test_root };
     } else {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         test_root = LexicalPath::join("/home/anon/Tests"sv, String::formatted("{}-tests", program_name.split_view('-').last())).string();
 #else
         char* serenity_source_dir = getenv("SERENITY_SOURCE_DIR");
@@ -158,7 +158,7 @@ int main(int argc, char** argv)
     }
 
     if (common_path.is_empty()) {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         common_path = "/home/anon/Tests/js-tests/test-common.js";
 #else
         char* serenity_source_dir = getenv("SERENITY_SOURCE_DIR");
diff --git a/Userland/Libraries/LibThreading/Mutex.h b/Userland/Libraries/LibThreading/Mutex.h
index 9ffb0613062..1ef71943142 100644
--- a/Userland/Libraries/LibThreading/Mutex.h
+++ b/Userland/Libraries/LibThreading/Mutex.h
@@ -24,7 +24,7 @@ public:
     Mutex()
         : m_lock_count(0)
     {
-#ifndef __serenity__
+#ifndef AK_OS_SERENITY
         pthread_mutexattr_t attr;
         pthread_mutexattr_init(&attr);
         pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
@@ -41,7 +41,7 @@ public:
     void unlock();
 
 private:
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     pthread_mutex_t m_mutex = PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP;
 #else
     pthread_mutex_t m_mutex;
diff --git a/Userland/Libraries/LibTimeZone/TimeZone.cpp b/Userland/Libraries/LibTimeZone/TimeZone.cpp
index 175bf5fd0f8..4bc21a27d7c 100644
--- a/Userland/Libraries/LibTimeZone/TimeZone.cpp
+++ b/Userland/Libraries/LibTimeZone/TimeZone.cpp
@@ -101,7 +101,7 @@ StringView current_time_zone()
         dbgln_if(TIME_ZONE_DEBUG, "Could not determine time zone from TZ environment: {}", time_zone);
     }
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     return system_time_zone();
 #else
     static constexpr auto zoneinfo = "/zoneinfo/"sv;
@@ -127,7 +127,7 @@ StringView current_time_zone()
 
 ErrorOr<void> change_time_zone([[maybe_unused]] StringView time_zone)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     TimeZoneFile time_zone_file("w");
 
     if (auto new_time_zone = canonicalize_time_zone(time_zone); new_time_zone.has_value())
diff --git a/Userland/Libraries/LibWeb/Loader/ResourceLoader.cpp b/Userland/Libraries/LibWeb/Loader/ResourceLoader.cpp
index c1270dc9174..845e9391eae 100644
--- a/Userland/Libraries/LibWeb/Loader/ResourceLoader.cpp
+++ b/Userland/Libraries/LibWeb/Loader/ResourceLoader.cpp
@@ -18,7 +18,7 @@
 #include <LibWeb/Platform/EventLoopPlugin.h>
 #include <LibWeb/Platform/Timer.h>
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
 #    include <serenity.h>
 #endif
 
@@ -127,7 +127,7 @@ static String sanitized_url_for_logging(AK::URL const& url)
 
 static void emit_signpost(String const& message, int id)
 {
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     auto string_id = perf_register_string(message.characters(), message.length());
     perf_event(PERF_EVENT_SIGNPOST, string_id, id);
 #else
diff --git a/Userland/Libraries/LibWebView/DOMTreeModel.cpp b/Userland/Libraries/LibWebView/DOMTreeModel.cpp
index 295cfdaa095..6096acc23d6 100644
--- a/Userland/Libraries/LibWebView/DOMTreeModel.cpp
+++ b/Userland/Libraries/LibWebView/DOMTreeModel.cpp
@@ -19,7 +19,7 @@ DOMTreeModel::DOMTreeModel(JsonObject dom_tree, GUI::TreeView* tree_view)
     , m_dom_tree(move(dom_tree))
 {
     // FIXME: Get these from the outside somehow instead of hard-coding paths here.
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     m_document_icon.set_bitmap_for_size(16, Gfx::Bitmap::try_load_from_file("/res/icons/16x16/filetype-html.png"sv).release_value_but_fixme_should_propagate_errors());
     m_element_icon.set_bitmap_for_size(16, Gfx::Bitmap::try_load_from_file("/res/icons/16x16/inspector-object.png"sv).release_value_but_fixme_should_propagate_errors());
     m_text_icon.set_bitmap_for_size(16, Gfx::Bitmap::try_load_from_file("/res/icons/16x16/filetype-unknown.png"sv).release_value_but_fixme_should_propagate_errors());
@@ -123,7 +123,7 @@ GUI::Variant DOMTreeModel::data(const GUI::ModelIndex& index, GUI::ModelRole rol
     auto type = node.get("type"sv).as_string_or("unknown"sv);
 
     // FIXME: This FIXME can go away when we fix the one below.
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     if (role == GUI::ModelRole::ForegroundColor) {
         // FIXME: Allow models to return a foreground color *role*.
         //        Then we won't need to have a GUI::TreeView& member anymore.
@@ -138,7 +138,7 @@ GUI::Variant DOMTreeModel::data(const GUI::ModelIndex& index, GUI::ModelRole rol
 #endif
 
     // FIXME: This FIXME can go away when the icons are provided from the outside (see constructor).
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
     if (role == GUI::ModelRole::Icon) {
         if (type == "document")
             return m_document_icon;
diff --git a/Userland/Shell/AST.cpp b/Userland/Shell/AST.cpp
index d26d17b72a7..ba79b546f95 100644
--- a/Userland/Shell/AST.cpp
+++ b/Userland/Shell/AST.cpp
@@ -625,7 +625,7 @@ void BarewordLiteral::highlight_in_editor(Line::Editor& editor, Shell& shell, Hi
             Line::Style bold = { Line::Style::Bold };
             Line::Style style = bold;
 
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
             if (runnable->kind == Shell::RunnablePath::Kind::Executable || runnable->kind == Shell::RunnablePath::Kind::Alias) {
                 auto name = shell.help_path_for({}, *runnable);
                 if (name.has_value()) {
diff --git a/Userland/Utilities/run-tests.cpp b/Userland/Utilities/run-tests.cpp
index f2ad1b23b7f..04e86bf2e9c 100644
--- a/Userland/Utilities/run-tests.cpp
+++ b/Userland/Utilities/run-tests.cpp
@@ -306,7 +306,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 #endif
 
     bool print_progress =
-#ifdef __serenity__
+#ifdef AK_OS_SERENITY
         true; // Use OSC 9 to print progress
 #else
         false;