diff --git a/lisp/subr.el b/lisp/subr.el
index f7eac75305..3fed3bc436 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -5453,10 +5453,15 @@ flatten-tree
 This always returns a list containing all the elements of TREE.
 \(flatten-tree \\='(1 (2 3 (4 5 (6))) 7))
 => (1 2 3 4 5 6 7)"
-  (cond ((null tree) nil)
-        ((consp tree) (append (flatten-tree (car tree))
-                              (flatten-tree (cdr tree))))
-        (t (list tree))))
+  (let (elems)
+    (setq tree (list tree))
+    (while (let ((elem (pop tree)))
+             (cond ((consp elem)
+                    (setq tree (cons (car elem) (cons (cdr elem) tree))))
+                   (elem
+                    (push elem elems)))
+             tree))
+    (nreverse elems)))
 
 ;; Technically, `flatten-list' is a misnomer, but we provide it here
 ;; for discoverability: