A spectrum analysis was carried out on sea level data from three stations in the Gulf of Finland, and one in the Archipelago sea. The spectra show amplification of the K1 and O1 tidal modes, but not of the M2 and S2 modes. The K1 and O1 modes have a period around 24 hours, which coincides with the estimated period of a possible fjord sieche in the Gulf of Finland. Despite a stronger astronomical forcing, M2 and S2 are much weaker in the spectra. This is consistent with the proposal by Witting (1911) of such a local fjord seiche. A two-dimensional linear shallow-water model of the Baltic Sea was then used to study these amplification fenomena in the Gulf of Finland. The results indicate a strong resonance for the periods 23 and 27 hours, well correlated with the K1 and O1 modes. This resonance is most naturally interpreted as a local fjord seiche, rather than as a global eigenmode of the whole Baltic Sea.