diff --git a/repo b/repo
index 8eda13581..9c70a2968 100755
--- a/repo
+++ b/repo
@@ -15,6 +15,8 @@ from pathlib import Path
 import subprocess
 import sys
 
+import gclient_utils
+
 # Some useful paths.
 DEPOT_TOOLS_DIR = Path(__file__).resolve().parent
 UPDATE_DEPOT_TOOLS = DEPOT_TOOLS_DIR / 'update_depot_tools'
@@ -45,6 +47,10 @@ def _UpdateDepotTools():
 
 
 def main(argv):
+    if gclient_utils.IsEnvCog():
+        print('"repo" is not supported in non-git environment.',
+              file=sys.stderr)
+        return 1
     # This is a bit hacky, but should be "good enough".  If repo itself gains
     # support for sync hooks, we could switch to that.
     if argv and argv[0] == 'sync':