We reconstruct Caribbean seawater temperatures from sclerosponge Sr/Ca ratios using a specimen of Ceratoporella nicholsoni that grew at 20 m below sea level in a reef cave at Jamaica. We sample the time interval from 1620 to 1745 A.D. with almost monthly resolution. This interval includes the Maunder sunspot minimum, one of the coldest periods of the Little Ice Age. Reconstructed annual temperature amplitudes are on the order of about 1¿C. The mean growth rate calculated from the annual Sr/Ca variations corresponds perfectly with U-Th-based growth rates. We find that the interannual climate variability is determined by El Ni¿o--Southern Oscillation and by a decadal signal, most likely originating from the tropical North Atlantic. On a multidecadal timescale the Maunder Minimum is characterized by a 1¿--2¿C cooling and reduced amplitudes of the interannual and decadal temperature variations.