Photosynthesis is used by plants, algae and bacteria to convert solar energy into stable chemical energy. The initial stages of this process-where light is absorbed and energy and electrons are transferred-are mediated by reaction centres composed of chlorophyll and carotenoid complexes(1). It has been previously shown that single small molecules can be used as functional components in electric(2-6) and optoelectronic circuits(7-10), but it has proved difficult to control and probe individual molecules for photovoltaic(11-13) and photoelectrochemical applications(14-16). Here, we show that the photocurrent generated by a single photosynthetic protein-photosystem I-can be measured using a scanning near-field optical microscope set-up. One side of the protein is anchored to a gold surface that acts as an electrode, and the other is contacted by a gold-covered glass tip. The tip functions as both counter electrode and light source. A photocurrent of similar to 10 pA is recorded from the covalently bound single-protein junctions, which is in agreement with the internal electron transfer times of photosystem I.