AbstractForaminiferal Mg/Ca has proven to be a powerful paleothermometer for reconstructing past sea‐surface temperature, which, among other applications, is a critical parameter for boron isotope reconstructions of past surface ocean pH and PCO₂. However, recent laboratory culture studies indicate seawater pH and the total dissolved inorganic carbon content (DIC) may both exert a significant additional control on foraminiferal Mg/Ca, likely influencing paleotemperature records as a result of seawater chemistry evolution on geologic timescales. In addition, the seawater Mg/Ca composition (Mg/Ca_sw) has been shown to reduce the sensitivity of foraminiferal Mg/Ca to temperature and possibly its sensitivity to the carbonate system as well. Here we present new Mg/Ca data from laboratory culture experiments with living planktic foraminifera— Globigerinoides ruber (p), Trilobatus sacculifer, and Orbulina universa— grown under a range of different pH and/or seawater DIC conditions and in low Mg/Ca_sw to mimic the chemical composition of the Paleocene ocean. We also conducted targeted [Ca] experiments to help define Mg/Ca_calcite–Mg/Ca_sw relationships for each species and conducted new pH experiments with G. bulloides. We find that pH effects on foraminiferal Mg/Ca are reduced or absent at Mg/Ca_sw = 1.5 mol/mol in all three species, and that T. sacculifer is generally insensitive to variable DIC and pH, making it the ideal species for Mg/Ca SST reconstructions back to 20 Ma. We apply our new T. sacculifer calibration to a Middle Miocene Mg/Ca record and provide recommendations for interpreting Mg/Ca records from extinct species.