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Earth's hydrological cycle was profoundly perturbed by massive carbon emissions during an ancient (56 Ma) global warming event referred to as the Paleocene‐Eocene thermal maximum (PETM). One approach to gaining valuable insight into the response of the hydrological cycle is to construct sea‐surface salinity (SSS) records that can be used to gauge changes in the rates of evaporation and precipitation during the PETM in such climatically sensitive areas as the circum‐Antarctic region. Here, we pair oxygen isotope (δ¹⁸O) and magnesium‐calcium (Mg/Ca) measurements to reconstruct PETM sea‐surface temperatures (SSTs) and δ¹⁸O composition of seawater (δ¹⁸O_sw) at austral Site 690 (Weddell Sea). Several discrepancies emerge between the δ¹⁸O‐ and Mg/Ca‐based SST records, with the latter indicating that the earliest PETM was punctuated by a short‐lived ~4°C increase in local SSTs. Conversion of the δ¹⁸O_swvalues to SSS reveals a ~4 ppt decrease ~50 ka after peak PETM warming at Site 690. This negative SSS (δ¹⁸O_sw) anomaly coincides with a prominent minimum in the planktic foraminifer δ¹⁸O record published for the Site 690 PETM section. Thus, our revised interpretation posits that this δ¹⁸O minimum signals a decrease in surface‐ocean δ¹⁸O_swfostered by a transient increase in mean annual precipitation in the Weddell Sea region. The results of this study corroborate the view that the poleward flux of atmospheric moisture temporarily increased during a distinctive stage of the PETM.