We investigated the effects of early marine diagenesis on the skeletal geochemistry of a Porites coral collected live offshore of Rabaul, Papua New Guinea. The lowermost 14 cm of the ~1.9 m coral core contains physical evidence of alteration, which is clearly visible in X-radiograph, SEM and petrography images. Abrupt and significant increases in coral δ18O and Sr/Ca, and decreases in coral Mg/Ca occur at the transition from unaltered to altered skeleton, all of which imply significant SST cooling, if the altered nature of the coral skeleton went unrecognized. Screening for the effects of early marine diagenesis should become a standard procedure in coral-climate studies.