Agattu is an island in the Near Islands in the western end of the Aleutian Islands. With a land area of Agattu is one of the largest uninhabited islands in the Aleutians. It is the second largest of the Near Islands, after Attu Island. It is volcanic and considerably mountainous. The treeless island has a tundra-like terrain which reaches a peak of above sea level. Its length is and width is .
Agattu has seven large seabird colonies, and an estimated population of 66,000 birds. About 1% of the global population of red-faced cormorants and tufted puffins nest on the island. Other inhabitants include rock sandpiper, red-necked phalarope, gray-crowned rosy finch and snow bunting. Aleutian cackling geese were reintroduced to the island after foxes were eliminated from the island in the 1970s. The elimination of the foxes on the island also made it possible for conservationists to reintroduce the Evermann's rock ptarmigan. In 2006 there were reports of at least 25 breeding pairs of the ptarmigan on the island.