diff --git a/dlls/user32/message.c b/dlls/user32/message.c
index c6daf319130..93a90ea8377 100644
--- a/dlls/user32/message.c
+++ b/dlls/user32/message.c
@@ -4229,6 +4229,16 @@ BOOL WINAPI KillSystemTimer( HWND hwnd, UINT_PTR id )
 }
 
 
+/**********************************************************************
+ *		IsGUIThread  (USER32.@)
+ */
+BOOL WINAPI IsGUIThread( BOOL convert )
+{
+    FIXME( "%u: stub\n", convert );
+    return TRUE;
+}
+
+
 /**********************************************************************
  *		GetGUIThreadInfo  (USER32.@)
  */
diff --git a/dlls/user32/user32.spec b/dlls/user32/user32.spec
index 072fed45593..5eb7154239e 100644
--- a/dlls/user32/user32.spec
+++ b/dlls/user32/user32.spec
@@ -436,7 +436,7 @@
 @ stdcall IsDialogMessageA(long ptr)
 @ stdcall IsDialogMessageW(long ptr)
 @ stdcall IsDlgButtonChecked(long long)
-# @ stub IsGUIThread
+@ stdcall IsGUIThread(long)
 @ stdcall IsHungAppWindow(long)
 # @ stub IsHungThread
 @ stdcall IsIconic(long)
diff --git a/include/winuser.h b/include/winuser.h
index 154c8da222a..83c99cb2061 100644
--- a/include/winuser.h
+++ b/include/winuser.h
@@ -4814,6 +4814,7 @@ WINUSERAPI BOOL        WINAPI IsDialogMessageA(HWND,LPMSG);
 WINUSERAPI BOOL        WINAPI IsDialogMessageW(HWND,LPMSG);
 #define                       IsDialogMessage WINELIB_NAME_AW(IsDialogMessage)
 WINUSERAPI UINT        WINAPI IsDlgButtonChecked(HWND,int);
+WINUSERAPI BOOL        WINAPI IsGUIThread(BOOL);
 WINUSERAPI BOOL        WINAPI IsHungAppWindow(HWND);
 WINUSERAPI BOOL        WINAPI IsIconic(HWND);
 WINUSERAPI BOOL        WINAPI IsMenu(HMENU);