An illumination button switch assembly for an electronic device includes a substrate, a switch element, a flexible and light-pervious supporting member, a light source, and a button cap. The switch element is disposed on the substrate to be triggered to generate a switching signal. The supporting member has a top end and a bottom end, a concave hollow portion is formed on the bottom end, and the bottom end is disposed above the switch element and is spaced from the switch element. The light source is disposed on the substrate to project light into the hollow portion. The button cap is fixed on the top end to be pressed to push the supporting member to contact the switch element with the bottom end. Since the button cap is light-pervious, the light passing through the hollow portion penetrates the button cap, thereby illuminating the button cap.