We have reconstructed mean annual coastal temperatures and seasonality during the Eocene (Priabonian; 36.6--40 Ma) using oxygen isotope profiles of shallow marine aragonitic gastropod shells (Gatuncillo Formation, Panama). These data provide a unique opportunity to document low-latitude coastal climates during a preglacial greenhouse period. Assuming shell oxygen isotope profiles primarily reflect changes in water temperature, our results indicate water temperatures in a middle to outer shelf setting (20--50 m depth) varied by 6¿--8¿C, and mean annual temperatures (MAT) were >26¿C. These temperatures are in agreement with pollen-based surface temperature reconstructions for the late Eocene and are consistent with other Eocene mollusk-based tropical coastal temperature estimates but are at odds with the few Eocene foraminiferal-based estimates of mean annual tropical sea surface temperatures (~17¿--22¿C).