diff --git a/reactos/include/psdk/winnt.h b/reactos/include/psdk/winnt.h
index 80878405e60..4b414207880 100644
--- a/reactos/include/psdk/winnt.h
+++ b/reactos/include/psdk/winnt.h
@@ -4321,18 +4321,26 @@ NTAPI
 RtlQueryDepthSList(
   _In_ PSLIST_HEADER ListHead);
 
-#ifndef _RTL_RUN_ONCE_DEF
-#define _RTL_RUN_ONCE_DEF
-
 #define RTL_RUN_ONCE_CHECK_ONLY 0x00000001UL
 #define RTL_RUN_ONCE_ASYNC 0x00000002UL
 #define RTL_RUN_ONCE_INIT_FAILED 0x00000004UL
 
+#define RTL_RUN_ONCE_INIT {0}
+
 typedef union _RTL_RUN_ONCE {
   PVOID Ptr;
 } RTL_RUN_ONCE, *PRTL_RUN_ONCE;
 
-#endif
+typedef DWORD WINAPI RTL_RUN_ONCE_INIT_FN(PRTL_RUN_ONCE, PVOID, PVOID*);
+typedef RTL_RUN_ONCE_INIT_FN *PRTL_RUN_ONCE_INIT_FN;
+
+NTSYSAPI
+DWORD
+WINAPI
+RtlRunOnceComplete(
+    PRTL_RUN_ONCE,
+    DWORD,
+    PVOID);
 
 #define RTL_CONDITION_VARIABLE_INIT {0}
 #define RTL_CONDITION_VARIABLE_LOCKMODE_SHARED 0x1