We demonstrate a low-cost, electrically tunable diffraction grating that is driven by a dielectric elastomer actuator. The angular tuning range of the polymer-based device is up to 118 mrad for the first diffracted order. The achievable grating period change of 32% is an improvement by more than a factor of 150 compared with existing analog tunable diffraction gratings based on hard materials. We show that in combination with a white light source, the presented diffraction grating can be used as a wavelength-adjustable luminous source. Such an illuminant has a potential application in inexpensive color displays that could reproduce all perceivable colors.