diff --git a/dlls/user32/defwnd.c b/dlls/user32/defwnd.c
index ba07ebb6b1..c31956c841 100644
--- a/dlls/user32/defwnd.c
+++ b/dlls/user32/defwnd.c
@@ -385,6 +385,18 @@ static LRESULT DEFWND_DefWinProc( HWND hwnd, UINT msg, WPARAM wParam, LPARAM lPa
         }
         break;
 
+    case WM_POPUPSYSTEMMENU:
+        {
+            /* This is an undocumented message used by the windows taskbar to
+               display the system menu of windows that belong to other processes. */
+            HMENU menu = GetSystemMenu(hwnd, FALSE);
+
+            if (menu)
+                TrackPopupMenu(menu, TPM_LEFTBUTTON|TPM_RIGHTBUTTON,
+                               LOWORD(lParam), HIWORD(lParam), 0, hwnd, NULL);
+            return 0;
+        }
+
     case WM_NCACTIVATE:
         return NC_HandleNCActivate( hwnd, wParam, lParam );
 
diff --git a/dlls/user32/spy.c b/dlls/user32/spy.c
index 41caf1d0b0..9fbd74b350 100644
--- a/dlls/user32/spy.c
+++ b/dlls/user32/spy.c
@@ -592,7 +592,8 @@ static const char * const MessageTypeNames[SPY_MAX_MSGNUM + 1] =
     "WM_PALETTEISCHANGING",
     "WM_PALETTECHANGED",
     "WM_HOTKEY",                /* 0x0312 */
-    NULL, NULL, NULL, NULL,
+    "WM_POPUPSYSTEMMENU",       /* 0x0313 */
+    NULL, NULL, NULL,
     "WM_PRINT",                 /* 0x0317 */
     "WM_PRINTCLIENT",           /* 0x0318 */
     "WM_APPCOMMAND",            /* 0x0319 */
diff --git a/dlls/user32/user_private.h b/dlls/user32/user_private.h
index da635b0ab4..40e57e1f09 100644
--- a/dlls/user32/user_private.h
+++ b/dlls/user32/user_private.h
@@ -82,6 +82,7 @@ static inline HLOCAL16 LOCAL_Free( HANDLE16 ds, HLOCAL16 handle )
 #define GET_DWORD(ptr) (*(const DWORD *)(ptr))
 
 #define WM_SYSTIMER	    0x0118
+#define WM_POPUPSYSTEMMENU  0x0313
 
 /* internal messages codes */
 enum wine_internal_message