This dataset includes grain size distribution, determined for 1-cm sediment slices of 5 short gravity-cores from the Kvarken Archipelago, eastern Gulf of Bothnia, northern Baltic Sea. Geochronology of the cores is constrained by the measurements of Cs-137 activity content. The sediments represent organic-rich brackish-water muds. The sample slices were freeze-dried and treated with excess hydrogen peroxide to remove organic matter prior to the analysis. Grain size distribution was analyzed down to 0.6 μm using a Micromeritics Sedigraph III 5120 Xray absorption sedimentation analyzer. Median grain size was calculated according to the geometric Folk and Ward (1957) graphical measures implemented in GRADISTAT 4.0 software.