A balloon illumination system, for illuminating a thin film balloon comprising a pair of thin film panels that are heat sealed together to define a balloon interior, the balloon having a balloon top and a balloon bottom, an apex at the balloon top, a mouth at the balloon bottom, and a flat film valve creating a sleeve having a sleeve width. The balloon illumination system comprises a tube having a tube circumference that is substantially equal to two times the sleeve width, a proximal end and a distal end. A light source is located in the tube midway between the proximal end and distal end. An electrical cable extends proximal through the tube from the light source. The distal end of the tube is inserted into the mouth of the tube and is extended through the sleeve until the distal end reaches the apex. Leakage is prevented by the tight fit between the tube and the sleeve. The light source is positioned in the balloon interior midway between the balloon top and balloon bottom. The light source is illuminated by connecting the electrical cable to a power source. By a further embodiment, the tube has an accordion portion having a plurality of transverse folds. The accordion portion is compressed while within the sleeve to pinch the sleeve therein and further prevent gases from leaving the balloon interior.