diff --git a/lib/isc/win32/include/isc/platform.h.in b/lib/isc/win32/include/isc/platform.h.in
index 41eecd94aa..e2bf50b681 100644
--- a/lib/isc/win32/include/isc/platform.h.in
+++ b/lib/isc/win32/include/isc/platform.h.in
@@ -9,8 +9,7 @@
  * information regarding copyright ownership.
  */
 
-#ifndef ISC_PLATFORM_H
-#define ISC_PLATFORM_H 1
+#pragma once
 
 /*****
  ***** Platform-dependent defines.
@@ -34,6 +33,10 @@
 typedef uint32_t socklen_t;
 #endif
 
+#ifndef thread_local
+#define thread_local __declspec(thread)
+#endif /* thread_local */
+
 /*
  * Limits
  */
@@ -97,5 +100,3 @@ typedef uint32_t socklen_t;
 #else
 #define LIBTESTS_EXTERNAL_DATA __declspec(dllimport)
 #endif
-
-#endif /* ISC_PLATFORM_H */
diff --git a/lib/isc/win32/ltdl.h b/lib/isc/win32/ltdl.h
index 16beb862ea..90bc10b097 100644
--- a/lib/isc/win32/ltdl.h
+++ b/lib/isc/win32/ltdl.h
@@ -19,7 +19,7 @@
 #define lt_dlsym(h, s) GetProcAddress(h, s)
 #define lt_dlclose(h)  FreeLibrary(h)
 
-_Thread_local LPSTR __dlerror_message[1024] = { 0 };
+__declspec(thread) LPSTR __dlerror_message[1024] = { 0 };
 
 static const char *
 lt_dlerror(void) {