An apparatus for illuminating the interior of a balloon includes a cylindrical plug with an integrally-formed radially extending integral flange insertable within the balloon neck. A light element is affixed to the plug and disposed within the neck. A battery is electrically connected to the light element and is typically mounted next to the light element, or includes the light element in a single assembly. Switching is provided to turn the lighting element on and off which, in the preferred embodiment is effected by inflating the balloon with the apparatus inserted into the balloon neck. The light element may be affixed to the head of the apparatus by magnetic means, by means of a screw-on cap, or by other well-known techniques.