The Shlisselburg Fortress or Oreshek Fortress is one of a series of fortifications built in Oreshek on Orekhovy Island in Lake Ladoga, near the modern city of Saint Petersburg in Russia. The first fortress was built in 1323. It was the scene of many conflicts between Russia and Sweden and changed hands between the two empires. During World War II, it was heavily damaged. Today it is part of the UNESCO World Heritage site Historic Centre of Saint Petersburg and Related Groups of Monuments.

Shlisselburg, formerly Oreshek (Орешек) (1323–1611) and Petrokrepost (Петрокрепость) (1944–1992), is a town in Kirovsky District, Leningrad Oblast, Russia, located at the head of the Neva River on Lake Ladoga, 35 kilometers (22 mi) east of St. Petersburg. Population: 13,170 (2010 Russian census); 12,401 (2002 Census); 12,589 (1989 Soviet census).