This study addresses the geological conundrum of giant salt basins by challenging the longstanding assumption that salt deposition necessarily occurs in brines in which the water depth is less than 300 m. We integrate regional observations, quantitative paleowater depth estimates, and numerical mass-balance modeling to illustrate that this assumption is inconsistent with observations from three giant salt deposits: the Mediterranean, northern South Atlantic, and northern Gulf of Mexico Basins. Our analysis indicates that these basins were very deep depressions before the onset of salt deposition, with a water-filled equivalent accommodation that exceeded 3500 m. Regional observations of pinch-out and downlap or onlap positions of these evaporites indicate a regional bathymetric relief of the top of salt with a slope of 0.5° to 0.7°. Our results demonstrate that these giant salt deposits can form by precipitation in ultradeep brine (>1000 m) settings and do not require complete desiccation to precipitate bittern salts. We propose an ultradeep basin, ultradeep brine class of salt basins, even if the salt was deposited during a major (∼1500 m) base-level drawdown like that in the Mediterranean. The mechanism of maintaining ultradeep brine conditions involves dynamic inflow of large fluxes of seawater into a restricted basin and simultaneous evaporation, which results in extremely rapid rates of salt deposition (∼4–>40 km/m.y.).