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Abstract The Paleocene‐Eocene Thermal Maximum (PETM, ∼56 million years ago) is among the best‐studied climatic warming events in Earth history and is often compared to projected anthropogenic climate change. The PETM is characterized by a rapid negative carbon isotope excursion and global temperature increase of 4–5°C, accompanied by changes in spatial patterns of evaporation and precipitation in the global hydrologic cycle. Recent climate model reconstructions suggest a regionally complex and non‐linear response of one important aspect of global hydrology: enhanced moisture flux from the low‐latitude ocean. In this study, we use the elemental and stable isotope geochemistry of surface‐dwelling planktic foraminifera from a low‐latitude Atlantic deep‐sea sedimentary record (IODP Site 1258) to quantify changes in sea‐surface temperature (SST) and salinity. Foraminiferal Mg/Ca and δ¹⁸O values are interpreted with a Bayesian forward proxy system model to reconstruct how SST and salinity changed over the PETM at this site. These temperature and salinity reconstructions are then compared to recent climate model simulations of Eocene warming. Our reconstructions indicate °C of warming, in excellent agreement with estimates from other tropical locations and modeled PETM warmth. The regional change in salinity is not as straightforward, demonstrating a slight decrease at extremepCO₂forcing (a reversal of the modeled sense of change under moderatepCO₂forcing) in both model and proxy reconstructions. The cause of this non‐linear response is unclear but may relate to increased South American continental runoff or shifts in the Inter‐Tropical Convergence Zone.