Just set background to null
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/button_text"
android:background="@null"/>
Or in code
button.setBackgroundResource(0);
or
button.setBackgroundDrawable(null);
Or you can define a style for a button in your theme so all buttons will have null background by default
<style name="AppTheme" parent="android:Theme">
<item name="android:buttonStyle">@style/NoBackgroundButtonStyle</item>
</style>
<style name="NoBackgroundButtonStyle" parent="android:Widget.Button">
<item name="android:background">@null</item>
</style>