Life on Earth is thought to have coevolved with the chemistry of the oceans and atmosphere, and the shift from an anoxic to an oxic world across the Archean-Proterozoic boundary represents a fundamental step in this process. In order to understand the relative influence of biological and geological factors on this transition, we must constrain key variables in seawater chemistry before the Great Oxidation Event (ca. 2500 Ma). We present a multielement (C-S-Fe-Mo) biogeochemical study of ca. 2662 Ma shales from the Hamersley Province in Western Australia. Our data reveal a sustained episode of Fe-limited pyrite formation under an anoxic and sulfidic (euxinic) water column. This is the oldest known occurrence of euxinia in Earth's history and challenges the paradigm of persistently Fe-rich Archean oceans. Bulk trace metal chemistry and preservation of strong mass-independent S isotope fractionations in sedimentary pyrites indicate that ocean euxinia was possible prior to oxidative weathering, suggesting that sulfidic waters may have been common throughout the Archean Eon. C-S-Fe systematics suggest that oxygenic photosynthesis was the primary source of organic carbon in the basin, and the absence of Mo enrichments highlights a potential link between inefficient nitrogen fixation and the delayed arrival of the Great Oxidation Event.