Halsön, or Halsö, is an island in the Bothnian Sea, in the Kvarken Archipelago of Finland. It is in the Korsnäs municipality.The island is marshy and wooded. It is used in part for forestry, and has some vacation cabins. The west of the island is a protected nature reserve, with an unusual environment of flads and gloe lakes that provide refuges for fish and birds.