The TOPEX/POSEIDON satellite has observed variations in global mean sea level with a precision of 4 mm at 10-day intervals since late 1992. During the 1997--1998 ENSO event, a 20 mm rise, and subsequent fall, of mean sea level was observed. These changes are well correlated with global mean sea surface temperature anomalies, which exhibit a similar response for every major ENSO event since 1981, suggesting the observed mean sea level change is mostly caused by thermal expansion. An Empirical Orthogonal Function analysis of the altimeter-derived sea level maps also suggests a connection with ENSO. We observed the same signal in global mean dynamic heights of the MOM2 ocean model and in anomalies of global mean precipitable water vapor. The presence of ENSO-variability in global mean sea level suggests that detecting the much smaller sea level variations associated with climate change will require at least a decade of precise altimeter measurements. ¿ 1999 American Geophysical Union