diff --git a/Kernel/Memory/PageDirectory.h b/Kernel/Memory/PageDirectory.h
index 2e09106e0b..8aecc3c049 100644
--- a/Kernel/Memory/PageDirectory.h
+++ b/Kernel/Memory/PageDirectory.h
@@ -12,6 +12,7 @@
 #include <AK/RefCounted.h>
 #include <AK/RefPtr.h>
 #include <Kernel/Forward.h>
+#include <Kernel/Locking/Spinlock.h>
 #include <Kernel/Memory/PhysicalPage.h>
 
 namespace Kernel::Memory {