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Abstract Twice in the Cryogenian Period (720–635 Ma), during the Sturtian and Marinoan glaciations, ice sheets extended to equatorial latitudes for millions of years. These climate extremes have been interpreted to record the Snowball climate state, in which all of Earth’s oceans were covered with ice. During a Snowball Earth, the hydrological cycle would have been curtailed and silicate weathering greatly reduced. In this scenario, deep ocean chemistry should have evolved toward mantle values through hydrothermal exchange at mid-ocean ridges. Specifically, seawater strontium isotopes (87Sr/86Sr) are predicted to exhibit unradiogenic mantle-like values. However, cap carbonates that overlie the Cryogenian glacial deposits have yielded radiogenic 87Sr/86Sr values similar to those of seawater prior to glaciation, inconsistent with the central geochemical prediction of the Snowball Earth hypothesis. Here we report the discovery of 87Sr/86Sr values of 0.7034 in marine carbonate and authigenic barite that rest directly above Sturtian glacial deposits in Dhofar, Oman. These values record either a local unradiogenic source or Snowball Earth deep-water values that have not been previously identified. If it is a global signal, these new data and geochemical modeling support an extreme Snowball Earth scenario with near-complete ice cover and define one of the largest geochemical perturbations to seawater in Earth history.