Petersburg is a city in and the county seat of Menard County, Illinois, United States, on the bluffs and part of the floodplain overlooking the Sangamon River. It is part of the Springfield, Illinois Metropolitan Statistical Area. The population was 2,260 at the 2010 census, and 2,226 at a 2018 estimate. Petersburg is located approximately two miles north of New Salem, the original settlement where Abraham Lincoln first settled, as he started his career.