We have studied soils and the plant cover of tombolos, which are isthmuses between the mainland and small islands on the coasts of the White and Baltic seas. Tombolos are characterized by complicated dynamics of sedimentation, which results in a strong heterogeneity of soils and associated plant cover. Typical gley and surface-gley marsh soils are formed on the Kolezhma plot on the White Sea coast. The vegetation corresponds to marsh meadows of the lower and middle levels and depends on salinity, acidity, and soil texture. Soils on the Gakkovo plot on the Baltic Sea coast are characterized by greater uniformity and, in general, a sandy texture. As compared to the White Sea, the effect of the Baltic Sea on the soil and plant cover is significantly smaller due to its slighter tidal dynamics.