diff --git a/src/core/common/heap.hpp b/src/core/common/heap.hpp
index 87011402f..529f6f2b9 100644
--- a/src/core/common/heap.hpp
+++ b/src/core/common/heap.hpp
@@ -36,6 +36,7 @@
 
 #include "openthread-core-config.h"
 
+#include <stddef.h>
 #include <string.h>
 
 namespace ot {