How to remove border from Textbox in WPF?

you can remove the border by just setting the BorderBrush property to Transparent.

Try BorderThickness="0"

<TextBox x:Name="SearchBox" BorderThickness="0" HorizontalAlignment="Left" Height="40" Width="296" Margin="10,0,0,0" TextWrapping="Wrap" VerticalAlignment="Center"  SelectionBrush="Black" Background="#00000000" Foreground="#FF5B5B5B" FontSize="25" FontFamily="Segoe UI Light" BorderBrush="#00000000" CaretBrush="#FF6C6C6C"/>