SCHLÜSSELBURG, a town of Russia, in the government of St Petersburg, situated on low ground surrounded by marshes, at the issue of the river Neva from Lake Ladoga, 40 m. by steamer E. of the city of St Petersburg. Pop. (1897) 5285. It was founded in 1323 by the Novgorodians, and though afterwards lost by Russia, was reconquered by Peter the Great in 1702. It has a cathedral and a fortress, built on an island in the Neva, which is now used as a political prison.