The process of mixing water masses that have differing temperatures, salinities, and pressures creates water that is typically denser than the average density of the source waters. Thus mixing the ocean, either along isopycnals or vertically, tends to decrease global sea level, even when no heat or salt is added to the ocean. This effect is small in the global ocean, but regional effects can be substantial. Two recommendations emerge from this study. First, numerical predictions of steric sea level rise should be designed to minimize sensitivity to erroneous isopycnal or vertical/diapycnal mixing rates that might over or under mix water masses. Second, retrospective analyses of steric sea level rise in historic data should avoid using gridded temperature and salinity fields to compute steric sea level change, since mapping smooths data in the same way that mixing does.