Hydrogen peroxide (H 2 O 2 ) plays diverse biological roles, and its effects in part depend on its spatiotemporal presence, in both intra-and extracellular contexts. A full understanding of the physiological effects of H 2 O 2 in both healthy and disease states is hampered by a lack of tools to controllably produce H 2 O 2 . Here, we address this issue by showing visible-light-induced production of exogenous H 2 O 2 by free-standing, gold-decorated silicon nanowires internalized in human umbilical vein endothelial cells. We further show that the photocatalytic production of H 2 O 2 is a general phenomenon of gold−silicon hybrid materials and is enhanced upon annealing.