diff --git a/include/mimalloc.h b/include/mimalloc.h
index a77f1dc5..0a4ae689 100644
--- a/include/mimalloc.h
+++ b/include/mimalloc.h
@@ -388,6 +388,7 @@ mi_decl_nodiscard mi_decl_export void* mi_new_reallocn(void* p, size_t newcount,
 // ---------------------------------------------------------------------------------------------
 #ifdef __cplusplus
 
+#include <cstddef>     // std::size_t
 #include <cstdint>     // PTRDIFF_MAX
 #if (__cplusplus >= 201103L) || (_MSC_VER > 1900)  // C++11
 #include <type_traits> // std::true_type