Petersburg (Tlingit: Gantiyaakw Séedi "Steamboat Channel") is a census-designated place (CDP) in and essentially the borough seat of Petersburg Borough, Alaska, United States. The population was 2,948 at the 2010 census, down from 3,224 in 2000.
The borough encompasses Petersburg and Kupreanof, plus mostly uninhabited areas stretching to the Canadian–American border and the southern boundary of the City and Borough of Juneau. While the city of Petersburg ceased to exist as a separate administrative entity (the borough assembly created a service area to assume operation of the former city's services), the tiny city of Kupreanof remains separate within the borough.
Tlingits from Kupreanof Island had long used a summer fish camp at the north end of Mitkof Island. Earlier cultures of indigenous people also used the island: remnants of fish traps and some petroglyphs have been carbon-dated back some 1,000 years.
European explorers to Mitkof Island encountered the Tlingit. In the nineteenth century, Peter Buschmann, a Norwegian immigrant, settled here, building a cannery, sawmill, docks and early structures. The settlement was named Petersburg after him, and it flourished as a fishing port. Icebergs from the nearby LeConte Glacier provided a source for cooling fish.
Petersburg originally incorporated as a town on April 2, 1910.