diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index bc05ce2c7fc..b2ed7180f39 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -2815,6 +2815,8 @@ Call FUN with two args (BEG and END) for each hunk."
 (defun diff--overlay-auto-delete (ol _after _beg _end &optional _len)
   (delete-overlay ol))
 
+(define-obsolete-function-alias 'diff-undo #'undo-ignore-read-only "31.1")
+
 ;;;###autoload
 (defcustom diff-add-log-use-relative-names nil
   "Use relative file names when generating ChangeLog skeletons.