Coral oxygen isotopes (δ18O) from the central equatorial Pacific provide monthly-resolved records of El Niño-Southern Oscillation (ENSO) activity over past centuries to millennia. However, calibration studies using in situ data to assess the relative contributions of warming and freshening to coral δ18O records are exceedingly rare. Furthermore, the fidelity of coral δ18O records under the most severe thermal stress events is difficult to assess. Here, we present six coral δ18O records and in situ temperature, salinity, and seawater δ18O data from Kiritimati Island (2°N, 15°W) spanning the very strong 2015/16 El Niño event. Local sea surface temperature (SST) anomalies of +2.4±0.4°C and seawater δ18O anomalies of -0.19±0.02‰ contribute to the observed coral δ18O anomalies of -0.58±0.05‰, consistent with a ~70% contribution from SST and ~30% from seawater δ18O. Our results demonstrate that Kiritimati coral δ18O records can provide reliable reconstructions even during the largest class of El Niño events.