Petersburg, the county seat of Grant County, is situated on the South Branch of the Potomac River near the Hardy County border, 30 miles from the Virginia line. The town is located at the junction of U.S. 220 and state routes 42 and 28. The 2010 population was 2,467.

Settled in 1745 as Lunice Creek, its name was changed to Petersburg, probably in honor of early merchant Jacob Peters, then to Grant Court House when it became the county seat in 1870, and finally back to Petersburg. In 1830, the population center of the United States was located nine miles west of town.

During the Civil War, numerous skirmishes occurred in and around the town. In April 1863, part of the Jones-Imobden Raid passed through the day before the Battle of Greenland Gap. Between August and December 1863, Union troops under Colonel James Mulligan built and occupied the earthen-and-timber Fort Mulligan at Petersburg to protect against Confederate attacks on the Baltimore & Ohio Railroad, particularly by guerrillas such as McNeill’s Rangers. At one point, more than 1,500 troops were stationed there. On January 31, 1864, in the face of an impending Confederate attack, Union commander Colonel Joseph Thoburn evacuated his troops from the fort. Confederate General Jubal Early launched an artillery barrage on Fort Mulligan before realizing it had been abandoned. The fort was not garrisoned again during the war. Today, it is one of the best preserved Civil War forts remaining in West Virginia and features an interpretive trail.

The city experienced significant growth spurts during the early 1900s and again from 1980 to 2010, when it reached its peak of 2,467. By 2020, the population had dropped slightly to 2,251. The town has a hospital, city library, volunteer fire department, newspaper, high school and elementary school, nine churches, several restaurants and motels, a variety of retail stores, and a city park. Winter weather is mild and summers warm but pleasant. Grant County Airport is nearby.

Widely known for its excellent trout fishing, Petersburg's proximity to the river, Smoke Hole Canyon, Spruce Knob, Seneca Rocks, and North Fork Mountain attracts many visitors. Annual celebrations include the Spring Mountain Festival and the Tri-County Fair (shared with Hardy and Pendleton counties). The Petersburg Wave, a powerful seasonal updraft, makes Petersburg popular with glider pilots.

Federal judge M. Blane Michael grew up in Petersburg.