diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 686cc84..2ed7399 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -396,6 +396,7 @@
 popup_dialog({what}, {options}) Number	create a popup window used as a dialog
 popup_filter_menu({id}, {key})  Number	filter for a menu popup window
 popup_filter_yesno({id}, {key}) Number	filter for a dialog popup window
+popup_findecho()		Number	get window ID of popup for `:echowin`
 popup_findinfo()		Number	get window ID of info popup window
 popup_findpreview()		Number	get window ID of preview popup window
 popup_getoptions({id})		Dict	get options of popup window {id}