We report here the light-driven activation of the molybdenum-iron-protein (MoFeP) of nitrogenase for substrate reduction independent of ATP hydrolysis and the iron-protein (FeP), which have been believed to be essential for catalytic turnover. A MoFeP variant labeled on its surface with a Ru-photosensitizer is shown to photocatalytically reduce protons and acetylene, most likely at its active site, FeMoco. The uncoupling of nitrogenase catalysis from ATP hydrolysis should enable the study of redox dynamics within MoFeP and the population of discrete reaction intermediates for structural investigations.