The Paleocene-Eocene Thermal Maximum (PETM) has been attributed to a rapid rise in greenhouse gas levels. If so, warming should have occurred at all latitudes, although amplified toward the poles. Existing records reveal an increase in high-latitude sea surface temperatures (SSTs) ($8\textdegree to 10\textdegree C$) and in bottom water temperatures ($4\textdegree to 5\textdegree C$). To date, however, the character of the tropical SST response during this event remains unconstrained. Here we address this deficiency by using paired oxygen isotope and minor element (magnesium/calcium) ratios of planktonic foraminifera from a tropical Pacific core to estimate changes in SST. Using mixed-layer foraminifera, we found that the combined proxies imply a$4\textdegree to 5\textdegree C$rise in Pacific SST during the PETM. These results would necessitate a rise in atmospheric pCO2to levels three to four times as high as those estimated for the late Paleocene.