Sagittal otoliths of the Peruvian sea catfish Galeichthys peruvianus were collected from the north coast of Peru during and after the 1997–1998 El Niño. The otoliths were analyzed via laser microprobe and micromilling techniques for oxygen isotope composition through ontogeny to document their use as an El Niño-Southern Oscillation (ENSO) proxy. Results were compared to theoretical calculations for the δ¹⁸O of otolith aragonite using measured sea surface temperatures (SST) and δ¹⁸O values for local seawater assuming equilibrium oxygen isotope fractionation was achieved. All otoliths recorded the 1997–1998 El Niño event as well as seasonal temperature variations. These ENSO events were recorded in otolith aragonite as significant negative excursions in δ¹⁸O that reflected the increased temperature of local marine waters. The combined otolith data were used to create a 10-year SST record, including ENSO events and local seasonal temperature variation, validating the use of otolith δ¹⁸O as a temperature proxy.