diff --git a/src/emacs/lean-mode.el b/src/emacs/lean-mode.el
index 1d0e50621..5c80c8d25 100644
--- a/src/emacs/lean-mode.el
+++ b/src/emacs/lean-mode.el
@@ -124,6 +124,7 @@ enabled and disabled respectively.")
   :syntax-table lean-syntax-table
   :abbrev-table lean-abbrev-table
   :group 'lean
+  (set (make-local-variable 'comment-start) "--")
   (set (make-local-variable 'comment-start-skip) "[-/]-[ \t]*")
   (set (make-local-variable 'comment-end) "")
   (set (make-local-variable 'comment-end-skip) "[ \t]*\\(-/\\|\\s>\\)")