Petersburg is a census-designated place in :Petersburg Borough, Alaska, United States. The population was 2,824 according to 2009 Census Bureau estimates.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 tiny city of Kupreanof remains separate within the borough.