The slowly growing coral Diploastrea heliopora affords a novel opportunity to obtain multicentury records of paleoclimate, including details of the interannual rhythms associated with the El Ni¿o--Southern Oscillation (ENSO) system. We examine climate variability in an ENSO-teleconnected region using new oxygen isotope records from D. heliopora, spanning most of the twentieth (1896--1998) and seventeenth (1622--1722) centuries, from the Mafia archipelago, Tanzania. The modern record demonstrates coherency with relevant instrumental and proxy time series, documents twentieth century warming, and displays significant power at ENSO periodicities. The seventeenth century record lacks any trend, exhibits interannual variance comparable to the modern record, and displays a pronounced interdecadal signal not evident in the twentieth century that correlates with other tropical and hemispheric climate records. We find no clear evidence of solar irradiance influence on interannual variability in the western Indian Ocean during these intervals, although the mean sea surface temperature appears to vary inversely with insolation.