diff --git a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Function.h b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Function.h
index 4eb7acd5897..0b49c0d76dd 100644
--- a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Function.h
+++ b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Function.h
@@ -43,6 +43,7 @@ private:
 
 struct FunctionArgument {
     StringView name;
+    size_t optional_arguments_group;
 };
 
 class FunctionDeclaration : public RefCounted<FunctionDeclaration> {
diff --git a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/Lexer.cpp b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/Lexer.cpp
index d4ba9901ed8..0a14990f25e 100644
--- a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/Lexer.cpp
+++ b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/Lexer.cpp
@@ -74,6 +74,8 @@ void tokenize_string(SpecificationParsingContext& ctx, XML::Node const* node, St
         { "("sv, TokenType::ParenOpen },
         { "+"sv, TokenType::Plus },
         { "?"sv, TokenType::QuestionMark },
+        { "]"sv, TokenType::SquareBracketClose },
+        { "["sv, TokenType::SquareBracketOpen },
     };
 
     LineTrackingLexer lexer(view, node->offset);
diff --git a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/TextParser.cpp b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/TextParser.cpp
index 638dbdb6222..845cad052ac 100644
--- a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/TextParser.cpp
+++ b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/TextParser.cpp
@@ -13,6 +13,8 @@ namespace JSSpecCompiler {
 
 void TextParser::save_error(Variant<TokenType, StringView, CustomMessage>&& expected)
 {
+    if (expected.has<TokenType>() && expected.get<TokenType>() == TokenType::Invalid)
+        return;
     if (m_max_parsed_tokens > m_next_token_index)
         return;
     if (m_max_parsed_tokens < m_next_token_index)
@@ -650,21 +652,45 @@ TextParseErrorOr<Vector<StringView>> TextParser::parse_qualified_name()
 // <function_arguments> :== '(' (<word> (, <word>)*)? ')'
 TextParseErrorOr<Vector<FunctionArgument>> TextParser::parse_function_arguments_in_declaration()
 {
-    Vector<FunctionArgument> arguments;
     TRY(consume_token_with_type(TokenType::ParenOpen));
+
+    Vector<FunctionArgument> arguments;
+    size_t optional_arguments_group = 0;
+
     while (true) {
-        if (arguments.is_empty()) {
-            auto argument = TRY(consume_token_with_one_of_types({ TokenType::ParenClose, TokenType::Identifier }));
-            if (argument.type == TokenType::ParenClose)
-                break;
-            arguments.append({ argument.data });
-        } else {
-            arguments.append({ TRY(consume_token_with_type(TokenType::Identifier)).data });
-        }
-        auto next_token = TRY(consume_token_with_one_of_types({ TokenType::ParenClose, TokenType::Comma }));
-        if (next_token.type == TokenType::ParenClose)
+        Token token = TRY(consume_token_with_one_of_types({
+            TokenType::SquareBracketOpen,
+            arguments.is_empty() ? TokenType::Identifier : TokenType::Comma,
+            !optional_arguments_group ? TokenType::ParenClose : TokenType::Invalid,
+            optional_arguments_group ? TokenType::SquareBracketClose : TokenType::Invalid,
+        }));
+
+        StringView identifier;
+
+        if (token.type == TokenType::SquareBracketClose) {
+            VERIFY(optional_arguments_group != 0);
+            for (size_t i = 1; i < optional_arguments_group; ++i)
+                TRY(consume_token_with_type(TokenType::SquareBracketClose));
+            TRY(consume_token_with_type(TokenType::ParenClose));
             break;
+        } else if (token.type == TokenType::ParenClose) {
+            VERIFY(optional_arguments_group == 0);
+            break;
+        } else if (token.type == TokenType::SquareBracketOpen) {
+            ++optional_arguments_group;
+            if (!arguments.is_empty())
+                TRY(consume_token_with_type(TokenType::Comma));
+            identifier = TRY(consume_token_with_type(TokenType::Identifier)).data;
+        } else if (token.type == TokenType::Comma) {
+            identifier = TRY(consume_token_with_type(TokenType::Identifier)).data;
+        } else {
+            VERIFY(token.type == TokenType::Identifier);
+            identifier = token.data;
+        }
+
+        arguments.append({ identifier, optional_arguments_group });
     }
+
     return arguments;
 }
 
diff --git a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/Token.h b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/Token.h
index 117ffb495db..67b4b4ebc34 100644
--- a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/Token.h
+++ b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Parser/Token.h
@@ -48,6 +48,8 @@ constexpr i32 closing_bracket_precedence = 18;
     F(Plus, 6, Invalid, Plus, Invalid, "plus")                                       \
     F(QuestionMark, 3, ReturnIfAbrubt, Invalid, Invalid, "question mark")            \
     F(SectionNumber, -1, Invalid, Invalid, Invalid, "section number")                \
+    F(SquareBracketClose, -1, Invalid, Invalid, Invalid, "']'")                      \
+    F(SquareBracketOpen, -1, Invalid, Invalid, Invalid, "'['")                       \
     F(String, -1, Invalid, Invalid, Invalid, "string literal")                       \
     F(Superscript, 4, Invalid, Power, Invalid, "subscript")                          \
     F(UnaryMinus, 3, Minus, Invalid, Invalid, "unary minus")                         \
diff --git a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Tests/spec-optional-arguments.xml b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Tests/spec-optional-arguments.xml
new file mode 100644
index 00000000000..f6ed3c5648f
--- /dev/null
+++ b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Tests/spec-optional-arguments.xml
@@ -0,0 +1,12 @@
+<!DOCTYPE inline_dtd[<!ENTITY nbsp " ">]>
+<specification>
+   <emu-clause id="1" aoid="TestOptionalArgumentsGroups1">
+      <h1><span class="secnum">1</span> TestOptionalArgumentsGroups1 ( [<var>a</var>, <var>b</var>[, <var>c</var>]] )</h1>
+      <emu-alg><ol><li>Return <emu-const>unused</emu-const>.</li></ol></emu-alg>
+   </emu-clause>
+
+   <emu-clause id="2" aoid="TestOptionalArgumentsGroups2">
+      <h1><span class="secnum">2</span> TestOptionalArgumentsGroups2 ( <var>a</var>, <var>b</var>[, <var>c</var>, <var>d</var>] )</h1>
+      <emu-alg><ol><li>Return <emu-const>unused</emu-const>.</li></ol></emu-alg>
+   </emu-clause>
+</specification>
diff --git a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Tests/spec-optional-arguments.xml.expectation b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Tests/spec-optional-arguments.xml.expectation
new file mode 100644
index 00000000000..b67d640eb61
--- /dev/null
+++ b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/Tests/spec-optional-arguments.xml.expectation
@@ -0,0 +1,11 @@
+===== AST after reference-resolving =====
+TestOptionalArgumentsGroups1([a, b, [c]]):
+TreeList
+  ReturnNode
+    Enumerator unused
+
+TestOptionalArgumentsGroups2(a, b, [c, d]):
+TreeList
+  ReturnNode
+    Enumerator unused
+
diff --git a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/main.cpp b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/main.cpp
index 950a239037f..9e86e6e2a2d 100644
--- a/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/main.cpp
+++ b/Meta/Lagom/Tools/CodeGenerators/JSSpecCompiler/main.cpp
@@ -74,11 +74,17 @@ template<>
 struct AK::Formatter<Vector<FunctionArgument>> : AK::Formatter<StringView> {
     ErrorOr<void> format(FormatBuilder& builder, Vector<FunctionArgument> const& arguments)
     {
+        size_t previous_optional_group = 0;
         for (size_t i = 0; i < arguments.size(); ++i) {
+            if (previous_optional_group != arguments[i].optional_arguments_group) {
+                previous_optional_group = arguments[i].optional_arguments_group;
+                TRY(builder.put_string("["sv));
+            }
             TRY(builder.put_string(arguments[i].name));
             if (i + 1 != arguments.size())
                 TRY(builder.put_literal(", "sv));
         }
+        TRY(builder.put_string(TRY(String::repeated(']', previous_optional_group))));
         return {};
     }
 };
diff --git a/Tests/JSSpecCompiler/test-runner.cpp b/Tests/JSSpecCompiler/test-runner.cpp
index 43b0d7d22dc..e4fb58e7902 100644
--- a/Tests/JSSpecCompiler/test-runner.cpp
+++ b/Tests/JSSpecCompiler/test-runner.cpp
@@ -50,6 +50,7 @@ const Array regression_tests = {
     TestDescription {
         .sources = {
             "spec-no-new-line-after-dot.xml"sv,
+            "spec-optional-arguments.xml"sv,
             "spec-parsing.xml"sv,
             "spec-single-function-simple.xml"sv,
         },