The Amundsen Sea is an arm of the Southern Ocean off Marie Byrd Land in western Antarctica. It is bounded by Thurston Island to the east and Cape Dart to the west. Named for the Norwegian polar explorer Roald Amundsen by the Norwegian expedition of 1928-29, under Captain Nils Larsen , while exploring this area in February, 1929.

The sea is mostly ice-covered, and the Thwaites Ice Tongue protrudes into it.