Ni-doped ZnS photocatalysts (Zn0.999Ni0.001 S) with a 2.4 eV energy gap showed activities for the reduction of nitrate and nitrite ions to nitrite, ammonia, and dinitrogen under visible light irradiation (lambda > 420 nm) in the presence of methanol as a reducing reagent. The reduction of nitrate ions competed with that of water to form dihydrogen. The concentration of nitrate ions and loading a platinum cocatalyst affected the selectivity for the reduction products of nitrate ions.