[2] At 0.49 square miles (1.27 km2), Petersburg is the second-smallest city by land area in the state of Michigan, after Clarkston.

The area was originally settled by Richard Peters as early as 1824, and he became the first postmaster when the Summerfield post office was established in April 1831.

Monroe natives Thomas Cole and Austin Wing formally established the community on land purchased from Peters in 1836.

[5] Petersburg is in western Monroe County, 17 miles (27 km) west of Monroe, the county seat, the same distance east of Adrian, and 21 miles (34 km) north of Toledo, Ohio.

[1] The River Raisin, a tributary of Lake Erie, forms part of the northwest edge of the city.

There were 476 housing units at an average density of 991.7 per square mile (382.9/km2).

22.5% of all households were made up of individuals, and 9.2% had someone living alone who was 65 years of age or older.

23.6% of all households were made up of individuals, and 9.9% had someone living alone who was 65 years of age or older.