diff --git a/tools/fmt-diff.py b/tools/fmt-diff.py
new file mode 100755
index 0000000..4e97f3a
--- /dev/null
+++ b/tools/fmt-diff.py
@@ -0,0 +1,31 @@
+#!/usr/bin/env python
+# Copyright (c) 2020, the R8 project authors. Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+
+import os
+import subprocess
+import sys
+
+import utils
+
+from subprocess import Popen, PIPE
+
+GOOGLE_JAVA_FORMAT_DIFF = os.path.join(
+    utils.THIRD_PARTY,
+    'google-java-format',
+    'google-java-format-google-java-format-1.7',
+    'scripts',
+    'google-java-format-diff.py')
+
+def main():
+  upstream = subprocess.check_output(['git', 'cl', 'upstream']).strip()
+  git_diff_process = Popen(['git', 'diff', '-U0', upstream], stdout=PIPE)
+  fmt_process = Popen(
+      ['python', GOOGLE_JAVA_FORMAT_DIFF, '-p1', '-i'],
+      stdin=git_diff_process.stdout)
+  git_diff_process.stdout.close()
+  fmt_process.communicate()
+
+if __name__ == '__main__':
+  sys.exit(main())