Electrodeposited thin films and nanoparticles of Ni3S2 are highly active, poison- and corrosion-resistant catalysts for oxygen reduction to water at neutral pH. In pH 7 phosphate buffer, Ni3S2 displays catalytic onset at 0.8 V versus the reversible hydrogen electrode, a Tafel slope of 109 mV decade(-1), and high faradaic efficiency for four-electron reduction of O-2 to water. Under these conditions, the activity and stability of Ni3S2 exceeds that of polycrystalline platinum and manganese, nickel, and cobalt oxides, illustrating the catalytic potential of pairing labile first-row transition metal active sites with a more covalent sulfide host lattice.