Button HorizontalAlignment: Stretch
<Window x:Class="WpfApplication1.Window1" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Title="ScrollViewerWindow" Height="222" Width="250"> <ScrollViewer> <StackPanel Orientation="Horizontal"> <Button Padding="10 4" HorizontalAlignment="Right">Fifth</Button> <Button Padding="10 4" HorizontalAlignment="Right">Fifth</Button> <Button Padding="10 4" HorizontalAlignment="Right">Fifth</Button> <Button HorizontalAlignment="Stretch">Sixth</Button> </StackPanel> </ScrollViewer> </Window>