Microatolls are discoid corals that have grown laterally because vertical growth is constrained by exposure at lowest tides. We demonstrate that a modern reef-flat Porites microatoll from Christmas (Kiritimati) Island preserves an oxygen isotope record of substantial sea surface temperature variations related to El Niño-Southern Oscillation (ENSO). We also show that a late Holocene fossil microatoll from the centre of the island contains interannual oxygen isotope variations over an approximate 20-year period. Three pronounced negative isotope anomalies attributed to warm El Niño events are superimposed on an annual cycle. El Niño events similar to those seen in recent decades appear to have been a feature of mid-Pacific climate in the late Holocene. Analysis of further microatolls offers a source of pre-instrumental El Niño tropical climate data. It may be possible to extend the NINO-C proxy to indicate intensity and frequency of ENSO over the past three millennia.