Earth's most severe climate changes occurred during global-scale "snowball Earth" glaciations, which profoundly altered the planet's atmosphere, oceans, and biosphere. Extreme rates of glacioeustatic sea level rise are predicted by the snowball Earth hypothesis, but supporting geologic evidence has been lacking. We use paleohydraulic analysis of wave ripples and tidal laminae in the Elatina Formation, Australia-deposited after the Marinoan glaciation similar to 635 million years ago-to show that water depths of 9 to 16 meters remained nearly constant for similar to 100 years throughout 27 meters of sediment accumulation. This accumulation rate was too great to have been accommodated by subsidence and instead indicates an extraordinarily rapid rate of sea level rise (0.2 to 0.27 meters per year). Our results substantiate a fundamental prediction of snowball Earth models of rapid deglaciation during the early transition to a supergreenhouse climate.