<jats:title>Abstract</jats:title> <jats:p>Between ~750 and 635 million years ago, during the Neoproterozoic era, the earth experienced at least two significant, possibly global, glaciations, termed “Snowball Earth.” While many studies have focused on the dynamics and the role of the atmosphere and ice flow over the ocean in these events, only a few have investigated the related associated ocean circulation, and no study has examined the ocean circulation under a thick (~1 km deep) sea ice cover, driven by geothermal heat flux. Here, a thick sea ice–flow model coupled to an ocean general circulation model is used to study the ocean circulation under Snowball Earth conditions. The ocean circulation is first investigated under a simplified zonal symmetry assumption, and (i) strong equatorial zonal jets and (ii) a strong meridional overturning cell are found, limited to an area very close to the equator. The authors derive an analytic approximation for the latitude–depth ocean dynamics and find that the extent of the meridional overturning circulation cell only depends on the horizontal eddy viscosity and β (the change of the Coriolis parameter with latitude). The analytic approximation closely reproduces the numerical results. Three-dimensional ocean simulations, with reconstructed Neoproterozoic continental configuration, confirm the zonally symmetric dynamics and show additional boundary currents and strong upwelling and downwelling near the continents.</jats:p>