Some of Earth's largest iron-nickel (Fe-Ni) sulfide ore deposits formed during the Archean and early Proterozoic. Establishing the origin of the metals and sulfur in these deposits is critical for understanding their genesis. Here, we present multiple sulfur isotope data implying that the sulfur in Archean komatiite-hosted Fe-Ni sulfide deposits was previously processed through the atmosphere and then accumulated on the ocean floor. High-temperature, mantle-derived komatiite magmas were then able to incorporate the sulfur from seafloor hydrothermal sulfide accumulations and sulfidic shales to form Neoarchean komatiite-hosted Fe-Ni sulfide deposits at a time when the oceans were sulfur-poor.