A 2D/3D switchable backlight unit and an image display device employing the same are provided. The 2D/3D switchable backlight unit includes a light source, a light guide plate in which light emitted from the light source is total-internal-reflected, and a switch array comprising a plurality of switches that selectively contact a first surface of the light guide plate and emit light by frustrated total internal reflection inside the light guide plate. In 2D mode, each of the switches contacts the first surface of the light guide plate. In 3D mode, some of the switches contact the first surface of the light guide plate.