Lake Petersburg is a census-designated place in Menard County, Illinois, United States.

[3] The unincorporated community centers on a small reservoir, which is used extensively by residents for recreation.

Lake Petersburg is southwest of the center of Menard County, surrounding a reservoir of the same name, built in a valley that flows to the Sangamon River less than a mile to the east.

Petersburg, the Menard county seat, is 2 miles (3 km) to the north.

According to the U.S. Census Bureau, the Lake Petersburg CDP has a total area of 2.08 square miles (5.39 km2), within which the reservoir occupies 0.28 square miles (0.73 km2), or 13.2% of the community's area.