Continuous seismic reflection profiling has revealed a 2000 m wide, almost circular depression in the crystalline bedrock of Tvären Bay. The depression extends more than 200 m below the present sea level. Originally, it was probably 250–300 m deep. The physical parameters of the structure, which forms a foreign element in the tectonic regime of the Baltic coastal area, are consistent with those of known impact structures. Inclined sedimentary rocks of an assumed Ordovician age are unevenly distributed within the depression, partially smoothening the topographic relief of the structure. A late Lower or early Middle Ordovician time for the meteorite impact is suggested. Drillings into the ‘wall’ surrounding the impact structure and into the sedimentary bedrock inside the depression are proposed as a further step in this investigation.