The hydrogen isotope composition of Early Proterozoic seawater has been indirectly determined on the basis of δD values measured in mafic-ultramafic samples from a hydrothermally altered layered flow from the Chukotat Group of the Lower Proterozoic (2.0–1.9 Ga) Cape Smith fold belt (northern Quebec). This flow and other mafic-ultramafic units of the fold belt represent some of the oldest relict oceanic crust identified on Earth. Measured δD values of whole-rock samples range from −53‰ to −40‰, matching closely values found in modern metavolcanic rocks and Phanerozoic ophiolites. A model of water-rock interaction for conditions of metamorphism deduced from mineral parageneses and Sr and O isotope data (T = 200-250 °C; water/rock ratio close to 1.0) indicates that the δD values of Chukotat flow samples reflect interaction with seawater that had a δD = 0‰ ± 20‰. This result implies that the hydrogen isotope composition of Early Proterozoic seawater was certainly similar to the modern value and that subduction and ridge processes that control the global water cycle have not changed significantly since the Early Proterozoic.