diff --git a/src/keymap.c b/src/keymap.c
index 782931fadf..94d49ac733 100644
--- a/src/keymap.c
+++ b/src/keymap.c
@@ -1180,27 +1180,8 @@ DEFUN ("command-remapping", Fcommand_remapping, Scommand_remapping, 1, 3, 0,
   return FIXNUMP (command) ? Qnil : command;
 }
 
-/* Value is number if KEY is too long; nil if valid but has no definition.  */
-/* GC is possible in this function.  */
-
-DEFUN ("lookup-key", Flookup_key, Slookup_key, 2, 3, 0,
-       doc: /* Look up key sequence KEY in KEYMAP.  Return the definition.
-A value of nil means undefined.  See doc of `define-key'
-for kinds of definitions.
-
-A number as value means KEY is "too long";
-that is, characters or symbols in it except for the last one
-fail to be a valid sequence of prefix characters in KEYMAP.
-The number is how many characters at the front of KEY
-it takes to reach a non-prefix key.
-KEYMAP can also be a list of keymaps.
-
-Normally, `lookup-key' ignores bindings for t, which act as default
-bindings, used when nothing else in the keymap applies; this makes it
-usable as a general function for probing keymaps.  However, if the
-third optional argument ACCEPT-DEFAULT is non-nil, `lookup-key' will
-recognize the default bindings, just as `read-key-sequence' does.  */)
-  (Lisp_Object keymap, Lisp_Object key, Lisp_Object accept_default)
+static Lisp_Object
+lookup_key_1 (Lisp_Object keymap, Lisp_Object key, Lisp_Object accept_default)
 {
   bool t_ok = !NILP (accept_default);
 
@@ -1240,6 +1221,46 @@ DEFUN ("lookup-key", Flookup_key, Slookup_key, 2, 3, 0,
     }
 }
 
+/* Value is number if KEY is too long; nil if valid but has no definition.  */
+/* GC is possible in this function.  */
+
+DEFUN ("lookup-key", Flookup_key, Slookup_key, 2, 3, 0,
+       doc: /* Look up key sequence KEY in KEYMAP.  Return the definition.
+A value of nil means undefined.  See doc of `define-key'
+for kinds of definitions.
+
+A number as value means KEY is "too long";
+that is, characters or symbols in it except for the last one
+fail to be a valid sequence of prefix characters in KEYMAP.
+The number is how many characters at the front of KEY
+it takes to reach a non-prefix key.
+KEYMAP can also be a list of keymaps.
+
+Normally, `lookup-key' ignores bindings for t, which act as default
+bindings, used when nothing else in the keymap applies; this makes it
+usable as a general function for probing keymaps.  However, if the
+third optional argument ACCEPT-DEFAULT is non-nil, `lookup-key' will
+recognize the default bindings, just as `read-key-sequence' does.  */)
+  (Lisp_Object keymap, Lisp_Object key, Lisp_Object accept_default)
+{
+  Lisp_Object found = lookup_key_1 (keymap, key, accept_default);
+
+  /* Since menu definitions sometimes use mixed case identifiers
+     (notably in old versions of `easy-menu-define'), also look for
+     the lowercase version.  */
+  if ((NILP (found) || NUMBERP (found))
+      && VECTORP (key) && EQ (AREF (key, 0), Qmenu_bar))
+    {
+      Lisp_Object new_key = Fmake_vector (make_fixnum (ASIZE (key)), Qnil);
+      for (int i = 0; i < ASIZE (key); i++)
+	  ASET (new_key, i, Fintern (Fdowncase (Fsymbol_name (AREF (key, i))),
+				     Qnil));
+      found = lookup_key_1 (keymap, new_key, accept_default);
+    }
+
+  return found;
+}
+
 /* Make KEYMAP define event C as a keymap (i.e., as a prefix).
    Assume that currently it does not define C at all.
    Return the keymap.  */
diff --git a/test/src/keymap-tests.el b/test/src/keymap-tests.el
index a9b0cb502d..19b4013b21 100644
--- a/test/src/keymap-tests.el
+++ b/test/src/keymap-tests.el
@@ -124,6 +124,17 @@ keymap-lookup-key/too-long
 ;; (ert-deftest keymap-lookup-key/accept-default ()
 ;;   ...)
 
+(ert-deftest keymap-lookup-key/mixed-case ()
+  (let ((map (make-keymap)))
+    (define-key map [menu-bar foo bar] 'foo)
+    (should (eq (lookup-key map [menu-bar foo bar]) 'foo))
+    (should (eq (lookup-key map [menu-bar Foo Bar]) 'foo))))
+
+(ert-deftest subr-test-lookup-keymap/with-spaces ()
+  (let ((map (make-keymap)))
+    (define-key map [menu-bar foo-bar] 'foo)
+    (should (eq (lookup-key map [menu-bar Foo\ Bar]) 'foo))))
+
 (ert-deftest describe-buffer-bindings/header-in-current-buffer ()
   "Header should be inserted into the current buffer.
 https://debbugs.gnu.org/39149#31"