diff --git a/gtk/gtkbbox.c b/gtk/gtkbbox.c
index 22ca734d8f..becdca2fba 100644
--- a/gtk/gtkbbox.c
+++ b/gtk/gtkbbox.c
@@ -349,6 +349,10 @@ gtk_button_box_set_layout (GtkButtonBox      *widget,
   if (priv->layout_style != layout_style)
     {
       priv->layout_style = layout_style;
+      if (priv->layout_style == GTK_BUTTONBOX_EXPAND)
+        gtk_box_set_homogeneous (GTK_BOX (widget), TRUE);
+      else
+        gtk_box_set_homogeneous (GTK_BOX (widget), FALSE);
       g_object_notify (G_OBJECT (widget), "layout-style");
       gtk_widget_queue_resize (GTK_WIDGET (widget));
     }
@@ -416,15 +420,25 @@ gtk_button_box_set_child_secondary (GtkButtonBox *widget,
                                     GtkWidget    *child,
                                     gboolean      is_secondary)
 {
+  GtkButtonBox *bbox;
+
   g_return_if_fail (GTK_IS_BUTTON_BOX (widget));
   g_return_if_fail (GTK_IS_WIDGET (child));
   g_return_if_fail (gtk_widget_get_parent (child) == GTK_WIDGET (widget));
 
+  bbox = GTK_BUTTON_BOX (widget);
+
   g_object_set_data (G_OBJECT (child),
                      GTK_BOX_SECONDARY_CHILD,
                      is_secondary ? GINT_TO_POINTER (1) : NULL);
   gtk_widget_child_notify (child, "secondary");
 
+  if (bbox->priv->layout_style == GTK_BUTTONBOX_EXPAND)
+    {
+      gtk_box_set_child_packing (GTK_BOX (bbox), child, TRUE, TRUE, 0,
+                                 is_secondary ? GTK_PACK_START : GTK_PACK_END);
+    }
+
   if (gtk_widget_get_visible (GTK_WIDGET (widget)) &&
       gtk_widget_get_visible (child))
     gtk_widget_queue_resize (child);
@@ -714,6 +728,7 @@ gtk_button_box_size_request (GtkWidget      *widget,
           case GTK_BUTTONBOX_START:
           case GTK_BUTTONBOX_END:
           case GTK_BUTTONBOX_CENTER:
+          case GTK_BUTTONBOX_EXPAND:
             if (orientation == GTK_ORIENTATION_HORIZONTAL)
               requisition->width = total_size + ((nvis_children - 1)*spacing);
             else
@@ -827,6 +842,13 @@ gtk_button_box_size_allocate (GtkWidget     *widget,
   bbox = GTK_BUTTON_BOX (widget);
   priv = bbox->priv;
 
+  if (priv->layout_style == GTK_BUTTONBOX_EXPAND)
+    {
+      GTK_WIDGET_CLASS (gtk_button_box_parent_class)->size_allocate (widget, allocation);
+      return;
+    }
+
+
   orientation = gtk_orientable_get_orientation (GTK_ORIENTABLE (widget));
   spacing = gtk_box_get_spacing (GTK_BOX (widget));
 
diff --git a/gtk/gtkenums.h b/gtk/gtkenums.h
index e87c983fc9..dbd74bebdb 100644
--- a/gtk/gtkenums.h
+++ b/gtk/gtkenums.h
@@ -162,6 +162,7 @@ typedef enum
  * @GTK_BUTTONBOX_END: Buttons are grouped towards the end of the box,
  *   (on the right for a HBox, or the bottom for a VBox).
  * @GTK_BUTTONBOX_CENTER: Buttons are centered in the box. Since 2.12.
+ * @GTK_BUTTONBOX_EXPAND: Buttons expand to fill the box. Since 3.12.
  *
  * Used to dictate the style that a #GtkButtonBox uses to layout the buttons it
  * contains. (See also: #GtkVButtonBox and #GtkHButtonBox).
@@ -172,7 +173,8 @@ typedef enum
   GTK_BUTTONBOX_EDGE,
   GTK_BUTTONBOX_START,
   GTK_BUTTONBOX_END,
-  GTK_BUTTONBOX_CENTER
+  GTK_BUTTONBOX_CENTER,
+  GTK_BUTTONBOX_EXPAND
 } GtkButtonBoxStyle;
 
 
diff --git a/tests/testbbox.c b/tests/testbbox.c
index 952684906a..8f2444db47 100644
--- a/tests/testbbox.c
+++ b/tests/testbbox.c
@@ -28,6 +28,7 @@ static const char* styles[] = { "GTK_BUTTONBOX_SPREAD",
 				"GTK_BUTTONBOX_START",
 				"GTK_BUTTONBOX_END",
 				"GTK_BUTTONBOX_CENTER",
+				"GTK_BUTTONBOX_EXPAND",
 				NULL};
 
 static const char* types[] = { "GtkHButtonBox",