diff --git a/lcl/interfaces/gtk2/gtk2proc.inc b/lcl/interfaces/gtk2/gtk2proc.inc
index 564ab0a933..5ee5bc5600 100644
--- a/lcl/interfaces/gtk2/gtk2proc.inc
+++ b/lcl/interfaces/gtk2/gtk2proc.inc
@@ -2367,7 +2367,7 @@ begin
       end;
     end;
 
-    if SysKey then
+    if SysKey or (ssAlt in ShiftState) then
       Flags := Flags or KF_ALTDOWN;
 
     CommonKeyData := KeyCode shl 16; // Not really scancode, but will do