diff --git a/cli/src/format.rs b/cli/src/format.rs
index d57724d721..9975332a93 100644
--- a/cli/src/format.rs
+++ b/cli/src/format.rs
@@ -1,4 +1,5 @@
-use std::path::PathBuf;
+use std::ffi::OsStr;
+use std::path::{Path, PathBuf};
 
 use crate::FormatMode;
 use bumpalo::collections::Vec;
@@ -39,7 +40,7 @@ fn flatten_directories(files: std::vec::Vec<PathBuf>) -> std::vec::Vec<PathBuf>
                                 let file_path = file.path();
                                 if file_path.is_dir() {
                                     to_flatten.push(file_path);
-                                } else if file_path.ends_with(".roc") {
+                                } else if is_roc_file(&file_path) {
                                     files.push(file_path);
                                 }
                             }
@@ -57,14 +58,19 @@ fn flatten_directories(files: std::vec::Vec<PathBuf>) -> std::vec::Vec<PathBuf>
                     error
                 ),
             }
-        } else {
-            files.push(path)
+        } else if is_roc_file(&path) {
+            files.push(path);
         }
     }
 
     files
 }
 
+fn is_roc_file(path: &Path) -> bool {
+    let ext = path.extension().and_then(OsStr::to_str);
+    return matches!(ext, Some("roc"));
+}
+
 pub fn format(files: std::vec::Vec<PathBuf>, mode: FormatMode) -> Result<(), String> {
     let files = flatten_directories(files);
 
diff --git a/cli/tests/cli_run.rs b/cli/tests/cli_run.rs
index 0a9213731d..edfc41ec70 100644
--- a/cli/tests/cli_run.rs
+++ b/cli/tests/cli_run.rs
@@ -73,7 +73,6 @@ mod cli_run {
     fn check_format_check_as_expected(file: &Path, expects_success_exit_code: bool) {
         let flags = &["--check"];
         let out = run_roc(&[&["format", file.to_str().unwrap()], &flags[..]].concat());
-
         if expects_success_exit_code {
             assert!(out.status.success());
         } else {
@@ -940,7 +939,7 @@ mod cli_run {
         // This fails, because "NotFormatted.roc" is present in this folder
         check_format_check_as_expected(&fixtures_dir("format"), false);
 
-        // This doesn't fail, since only "Formatted.roc" is present in this folder
+        // This doesn't fail, since only "Formatted.roc" and non-roc files are present in this folder
         check_format_check_as_expected(&fixtures_dir("format/formatted_directory"), true);
     }
 }
diff --git a/cli/tests/fixtures/format/NotARocFile.txt b/cli/tests/fixtures/format/NotARocFile.txt
new file mode 100644
index 0000000000..ea65efc5d0
--- /dev/null
+++ b/cli/tests/fixtures/format/NotARocFile.txt
@@ -0,0 +1 @@
+This is not a .roc file, and should be ignored by the formatter.
\ No newline at end of file
diff --git a/cli/tests/fixtures/format/formatted_directory/NestedNotARocFile.txt b/cli/tests/fixtures/format/formatted_directory/NestedNotARocFile.txt
new file mode 100644
index 0000000000..ea65efc5d0
--- /dev/null
+++ b/cli/tests/fixtures/format/formatted_directory/NestedNotARocFile.txt
@@ -0,0 +1 @@
+This is not a .roc file, and should be ignored by the formatter.
\ No newline at end of file
diff --git a/cli/tests/fixtures/format/formatted_directory/NotARocFile b/cli/tests/fixtures/format/formatted_directory/NotARocFile
new file mode 100644
index 0000000000..63b7992a35
--- /dev/null
+++ b/cli/tests/fixtures/format/formatted_directory/NotARocFile
@@ -0,0 +1,3 @@
+This is not a .roc file, and should be ignored by the formatter.
+
+This file does not have an extension, to ensure the formatter does not simply test with `ends_with(".roc")`
\ No newline at end of file
diff --git a/cli/tests/fixtures/format/formatted_directory/ignored/NotARocFile.txt b/cli/tests/fixtures/format/formatted_directory/ignored/NotARocFile.txt
new file mode 100644
index 0000000000..ea65efc5d0
--- /dev/null
+++ b/cli/tests/fixtures/format/formatted_directory/ignored/NotARocFile.txt
@@ -0,0 +1 @@
+This is not a .roc file, and should be ignored by the formatter.
\ No newline at end of file