diff --git a/webrtc/base/arraysize.h b/webrtc/base/arraysize.h
index 93ea0f6de..0bb0a62ed 100644
--- a/webrtc/base/arraysize.h
+++ b/webrtc/base/arraysize.h
@@ -11,6 +11,8 @@
 #ifndef WEBRTC_BASE_ARRAYSIZE_H_
 #define WEBRTC_BASE_ARRAYSIZE_H_
 
+#include <stddef.h>
+
 // This file defines the arraysize() macro and is derived from Chromium's
 // base/macros.h.