Petersburg is a small unincorporated community and census-designated place in southeastern Springfield Township, Mahoning County, Ohio, United States. The population was 405 at the 2020 census. It is part of the Youngstown–Warren metropolitan area. Petersburg is located at the intersection of Ohio State Routes 170 and 617. It is skirted by Interstate 76, the far eastern end of the Ohio Turnpike. The community was founded in the 1790s by German immigrants from Pennsylvania, which it neighbors.