diff --git a/git_cl.py b/git_cl.py
index b1cf7fa26..a2d9c628e 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -2306,7 +2306,7 @@ def CMDowners(parser, args):
 @subcommand.usage('[files or directories to diff]')
 def CMDformat(parser, args):
   """Runs clang-format on the diff."""
-  CLANG_EXTS = ['.cc', '.cpp', '.h']
+  CLANG_EXTS = ['.cc', '.cpp', '.h', '.mm']
   parser.add_option('--full', action='store_true',
                     help='Reformat the full content of all touched files')
   parser.add_option('--dry-run', action='store_true',