Investigation of free sea level oscillations is important for understanding of specifics of oceanographic processes and for their identification. Based on numerical experiments with the 3-D INMOM hydrodynamic model, we demonstrated that after cessation of atmospheric forcing, the water masses of the Baltic Sea return to equilibrium state as in the form of barotropic progressive–standing waves with 13–44 h periods, as in the form of baroclinic low-frequency waves with periods of 89 and 358 d.