<p>For the severe low-latitude “snowball Earth” glaciations, glacial deposits occurring on all continents is well-established. However, cold, salty, ice-covered oceans—a salient prediction of snowball Earth—is difficult to establish geologically. Here we demonstrate that anomalously high iron isotope values (δ⁵⁶Fe) of snowball iron formation—never observed in earlier anoxic Archaean oceans—can be attributed to additional temperature-dependent fractionation in extremely cold brine pools in the snowball ocean. Experiments and modeled fractionations relevant to the precipitation of iron formation demonstrate temperature-dependent δ⁵⁶Fe fractionation, where colder temperatures correspond with more positive δ⁵⁶Fe. Assuming the ~ 0.9‰ differential in δ⁵⁶Fe values of snowball iron formation in excess of those preceding the Great Oxidation Event is due to temperature-dependent fractionation, we calculate that the temperature of the iron formation brine pools was –15 ± 7°C. Such cold snowball brine pools, colder than those in Antarctic margins today, represent Earth’s coldest recorded ocean temperatures.</p>