El Niño–Southern Oscillation (ENSO) powers global interannual climate variability through changes in trade wind strength, temperature and salinity anomalies, sea level, and atmospheric circulation patterns. ENSO variability is well characterized in modern times, but instrumental records cannot fully describe natural ENSO variability due to the imprint of anthropogenic climate forcing. ENSO activity may also be affected by solar variability, but the response of ENSO to such changes is difficult to predict. We constructed a continuous, monthly resolved, spliced fossil Porites coral δ18O and Sr/Ca record from Misima Island, Papua New Guinea, in the Western Pacific Warm Pool, spanning 233 yr (1411–1644 CE [Common Era]). The Misima coral record indicates that the surface ocean in this region experienced a small change in hydrologic balance with no change in temperature, extended periods of quiescence in El Niño activity, and no change in average amplitudes of El Niño events relative to signals captured in regional modern records. The reduced El Niño variability occurs during a known change in solar forcing, the initiation of the Little Ice Age. However, there is no clear relationship between the timing of changes in solar forcing and ENSO activity, implying that ENSO variability changes arise from internal dynamics. The century-scale switch between active and inactive El Niño states has not previously been recorded, and provides a new baseline for climate models and reconstructions.

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