Mitkof Island is an island in the Alexander Archipelago in southeast Alaska between Kupreanof Island to the west and the Alaska mainland to the east. It is about 16 km (9.9 mi) wide and 28 km (17 mi) long with a land area of 539.7 km sq (208.4 sq mi), making it the 30th largest island in the United States.
The island is relatively Flat with numerous muskegs. The highest point is Crystal Mountain (3,317 feet).
The city of Petersburg is on the north end of the island. The total population of the island was 3,364 at the 2000 census, almost all of it in the city of Petersburg.