diff --git a/lean4-mode/lean4-syntax.el b/lean4-mode/lean4-syntax.el
index c237b1e8a9..9c752b9a0b 100644
--- a/lean4-mode/lean4-syntax.el
+++ b/lean4-mode/lean4-syntax.el
@@ -22,7 +22,7 @@
     "try" "catch" "finally" "where" "rec" "forall" "Pi" "fun"
     "exists" "if" "then" "else" "assume" "from" "init_quot" "return"
     "mutual" "def" "run_cmd" "declare_syntax_cat" "syntax" "macro_rules" "macro"
-    "initialize")
+    "initialize" "builtin_initialize")
   "lean keywords ending with 'word' (not symbol)")
 (defconst lean4-keywords1-regexp
   (eval `(rx word-start (or ,@lean4-keywords1) word-end)))