The climate dynamical controls on rainfall oxygen isotope values (δ ¹⁸O_rain) are poorly constrained in most areas. In Central America, rainfall variations are associated with sea surface temperature anomalies in the Pacific and Atlantic Oceans, yet the response in δ ¹⁸O_rain has not been adequately investigated. Analysis of a 30-year time series of δ ¹⁸O_rain in Panama was used to test the hypothesis that δ ¹⁸O_rain variations are forced by anomalies in rainfall and sea surface temperatures (SSTAs) in the bordering tropical oceans. The results show that δ ¹⁸O_rain values in Panama are positively (negatively) correlated with SSTAs in the Pacific Ocean (Caribbean Sea). The data are consistent with a dominant 'amount effect' whereby lowest δ ¹⁸O values are associated with a northerly position of the Intertropical Convergence Zone when regional rainfall is high. The source effect (Caribbean vs. Pacific), as constrained by wind direction, does not appear to be an important control on the temporal variations in δ ¹⁸O_rain.