Graphene has recently been shown to be permeable to thermal protons(1), the nuclei of hydrogen atoms, which sparked interest in its use as a proton-conducting membrane in relevant technologies(1)(-4). However, the influence of light on proton permeation remains unknown. Here we report that proton transport through Pt-nanoparticle-decorated graphene can be enhanced strongly by illuminating it with visible light. Using electrical measurements and mass spectrometry, we find a photoresponsivity of similar to 10(4) A W-1, which translates into a gain of similar to 10(4) protons per photon with response times in the microsecond range. These characteristics are competitive with those of state-of-the-art photodetectors that are based on electron transport using silicon and novel two-dimensional materials(5)(-7). The photo-proton effect could be important for graphene's envisaged use in fuel cells and hydrogen isotope separation. Our observations may also be of interest for other applications such as light-induced water splitting, photocatalysis and novel photodetectors.