Analysis of TOPEX/Poseidon satellite altimeter data indicates that the global mean sea level variation has a clear seasonal signal with an amplitude of about 2 to 3 mm, along with a long term drift. This seasonal variation is associated with mass redistribution within the global hydrological cycle plus steric thermal contributions. We investigate seasonal variations of water vapor in the atmosphere and water storage on land using both assimilated atmospheric models and climatological data, and to estimate the corresponding global mean sea level changes. The predicted seasonal global mean sea level changes are then compared with the seasonal variabilities observed by TOPEX/Poseidon altimetry data, after the latter are corrected for the steric effect using a simplified thermal model derived from the NOAA World Ocean Atlas 1994. The good agreement in both amplitude and phase indicates that the T/P altimeter may provide key information for the global water mass budget by placing observational constraints on the mass budget variations predicted by global atmospheric and hydrological models.