diff --git a/lcl/dialogs.pp b/lcl/dialogs.pp
index 65f39e5e7c..6ac54612e5 100644
--- a/lcl/dialogs.pp
+++ b/lcl/dialogs.pp
@@ -292,18 +292,20 @@ type
     cdPreventFullOpen,  // Disables the Define Custom Color button.
     cdShowHelp,         // Causes the dialog box to display the Help button.
     cdSolidColor,       // Causes the dialog box to display only solid colors in the set of basic colors.
-    cdAnyColor          // Causes the dialog box to display all available colors in the set of basic colors.
+    cdAnyColor,         // Causes the dialog box to display all available colors in the set of basic colors.
+    cdShowAlphaChannel  // Shows the AlphaChannel control (GTK2, QT, QT5, QT6)
     );
   TColorDialogOptions = set of TColorDialogOption;
 
 const
-  DefaultColorDialogOptions = [cdFullOpen]; //for backwards compatibility, probably not Delphi compatible
+  DefaultColorDialogOptions = [cdFullOpen, cdShowAlphaChannel]; //for backwards compatibility, probably not Delphi compatible
 
   { TColorDialog }
 type
 
   TColorDialog = class(TCommonDialog)
   private
+    FAlphaChannel: Byte;
     FColor: TColor;
     FCustomColors: TStrings;
     FOptions: TColorDialogOptions;
@@ -318,6 +320,7 @@ type
     destructor Destroy; override;
   published
     property Title;
+    property AlphaChannel: Byte read FAlphaChannel write FAlphaChannel default 255;
     property Color: TColor read FColor write FColor;
     // entry looks like ColorA = FFFF00 ... ColorX = C0C0C0
     property CustomColors: TStrings read FCustomColors write SetCustomColors;
diff --git a/lcl/include/lclcolordialog.inc b/lcl/include/lclcolordialog.inc
index 37af28b189..1019b6992a 100644
--- a/lcl/include/lclcolordialog.inc
+++ b/lcl/include/lclcolordialog.inc
@@ -48,6 +48,7 @@ end;
 constructor TColorDialog.Create(TheOwner: TComponent);
 begin
   inherited Create(TheOwner);
+  FAlphaChannel := 255;
   FCustomColors := TStringList.Create;
   // add default colors
   GetColorValues(@AddDefaultColor);
diff --git a/lcl/interfaces/gtk2/gtk2wsdialogs.pp b/lcl/interfaces/gtk2/gtk2wsdialogs.pp
index 1264ae6cc7..d522cec238 100644
--- a/lcl/interfaces/gtk2/gtk2wsdialogs.pp
+++ b/lcl/interfaces/gtk2/gtk2wsdialogs.pp
@@ -441,6 +441,7 @@ var
   // colordialog
   colorsel : PGtkColorSelection;
   newColor : TGdkColor;
+  newAlpha : Word;
   // fontdialog
   FontName : String;
   ALogFont  : TLogFont;
@@ -542,6 +543,8 @@ begin
   begin
     colorSel := PGtkColorSelection(PGtkColorSelectionDialog(FPointer)^.colorsel);
     gtk_color_selection_get_current_color(colorsel, @newColor);
+    newAlpha := gtk_color_selection_get_current_alpha(colorsel);
+    TColorDialog(theDialog).AlphaChannel := newAlpha and $FF; //equivalent to divide by 0x0101
     TColorDialog(theDialog).Color := TGDKColorToTColor(newcolor);
     {$IFDEF VerboseColorDialog}
     DebugLn('gtkDialogOKclickedCB ',DbgS(TColorDialog(theDialog).Color));
@@ -1278,16 +1281,24 @@ end;
 class procedure TGtk2WSCommonDialog.ShowModal(const ACommonDialog: TCommonDialog);
 var
   GtkWindow: PGtkWindow;
+  colorsel: PGtkColorSelection;
+  ColorDialog: TColorDialog;
+  CurrentAlpha: Word;
 begin
   ReleaseMouseCapture;
   GtkWindow:={%H-}PGtkWindow(ACommonDialog.Handle);
   gtk_window_set_title(GtkWindow,PChar(ACommonDialog.Title));
   if ACommonDialog is TColorDialog then
   begin
+    ColorDialog := TColorDialog(ACommonDialog);
     SetColorDialogColor(PGtkColorSelectionDialog(GtkWindow),
-                        TColorDialog(ACommonDialog).Color);
+                        ColorDialog.Color);
     SetColorDialogPalette(PGtkColorSelectionDialog(GtkWindow),
-      TColorDialog(ACommonDialog).CustomColors);
+      ColorDialog.CustomColors);
+    colorsel := PGtkColorSelection(PGtkColorSelectionDialog(GtkWindow)^.colorsel);
+    gtk_color_selection_set_has_opacity_control(colorsel, (cdShowAlphaChannel in ColorDialog.Options));
+    CurrentAlpha := Word(ColorDialog.AlphaChannel) * $0101;
+    gtk_color_selection_set_current_alpha(colorsel, CurrentAlpha);
   end;
 
   gtk_window_set_position(GtkWindow, GTK_WIN_POS_CENTER);