The postglacial rebound (PGR) in Fennoscandia is an attractive signal for the detection of time-variable gravity with CHAMP and GRACE. However, the separation from other geophysical signals is a challenge. We study the influence of oceanographic variations of the Baltic Sea. Using a precise high-resolution oceanographic model we analyze its water mass variations and the resulting geoid signal. The geoid signal is similar to the postglacial rebound signal in spatial pattern and in magnitude. Hence we show that the PGR. detection from satellite gravity data will be corrupted by Baltic Sea oceanographic variations unless they are accounted for. Our study suggests to correct for the Baltic Sea signal using external data.