[2] It is part of the Skagerrak sea and is located in Strömstad and Tanum municipalities in Bohuslän, Västra Götaland County, Sweden.

Kosterhavet National Park is home to Sweden's largest seal colony.

The Kosterfjord (English: Koster Fjord)[6][7] is 200 m (660 ft) deep with relatively low temperature – 5 to 7 °C (41 to 45 °F) – and high salinity (about 35%).

Rare seabirds, such as Arctic terns and skuas, along with a large population of harbor seals have their habitat here.

Plaice, cod and sea trout breed in the more shallow waters closer to the shore.