diff --git a/runtime/syntax/nginx.vim b/runtime/syntax/nginx.vim
index d036c12..ad8bdbf 100644
--- a/runtime/syntax/nginx.vim
+++ b/runtime/syntax/nginx.vim
@@ -2,6 +2,7 @@
 " Language: nginx.conf
 " Maintainer: Chris Aumann <me@chr4.org>
 " Last Change: Nov 25, 2023
+" 2025 Jun 08 by Vim Project: improve syntax (#17458)
 
 if exists("b:current_syntax")
   finish
@@ -389,6 +390,7 @@
 syn keyword ngxDirective proxy_buffers
 syn keyword ngxDirective proxy_busy_buffers_size
 syn keyword ngxDirective proxy_cache
+syn keyword ngxDirective proxy_cache_background_update
 syn keyword ngxDirective proxy_cache_bypass
 syn keyword ngxDirective proxy_cache_convert_head
 syn keyword ngxDirective proxy_cache_key