Petersburg is a city in 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 2,299 at the 2000 census, and 2,185 at a 2009 estimate. It is the county seat of Menard County. Petersburg is located approximately two miles north of New Salem, the original settlement where Abraham Lincoln first settled when he came to Illinois.