We analyzed primary fluid inclusions in halite from marine evaporites in the ca. 830 Ma Browne Formation of the Officer Basin in Western Australia using the cryogenic scanning electron microscopy–energy dispersive X-ray spectrometry analysis technique. The concentrations of the major ions, except K+ and possibly SO42−, fall within the range of Phanerozoic seawater. This is the first direct measurement of the composition of mid-Neoproterozoic seawater, and extends present-day knowledge of seawater chemistry by ∼300 m.y. Our estimates suggest that mid-Neoproterozoic marine sulfate concentrations were lower (∼90%) than modern values. By the terminal Neoproterozoic, fluid inclusions in halite and evaporite mineralogy indicate seawater sulfate levels rose significantly, to 50%–80% of modern concentrations, which parallels increases in atmospheric and oceanic oxygen.