Stora Hamnskär is an island in the northeast of the Swedish sector of the Bothnian Bay, in the Haparanda archipelago.

[1] All of the islands in the Haparanda archipelago have emerged in the last 1,500 years or so, as the bed of the bay has risen due to post-glacial rebound following the last ice age.

[3] The isthmus between Stora Hamnskär and the Prokko Peninsula is mainly covered with grasslands, with flora that shows the influence of limestone.

[3] About 30.3 hectares (75 acres) of the island has been chosen for inclusion in Natura 2000 because it is abundant in Siberian Primrose (Primula nutans), a species listed in the Habitats directive.

The environment on Stora Hepokari is unstable, because uplift eliminates suitable terrain while new areas appear.