PA 551 begins at an intersection with PA 18 in the city of Beaver Falls in Beaver County, heading west on two-lane undivided 49th Street Extended. The road passes between commercial areas to the north and homes to the south, crossing under Norfolk Southern's Fort Wayne Line and heading into the borough of West Mayfield, becoming Wallace Run Road. The route heads into wooded areas with some homes, crossing Wallace Run into the borough of Big Beaver. PA 551 continues northwest along the northern bank of Wallace Run, coming to an interchange with I-376 that has a toll plaza on the ramps to and from the northbound direction of I-376. The road heads into a mix of farmland and woodland with some residences, curving more to the west. The route heads southwest and crosses into Chippewa Township, turning northwest and briefly passing through Big Beaver again prior to entering Darlington Township. PA 551 curves west and heads into South Beaver Township, where it turns north onto Old Darlington Road and comes to an intersection with PA 168. Here, PA 168 turns north to form a concurrency with the route, crossing the North Fork Little Beaver Creek into Darlington Township. The road heads into the borough of Darlington and becomes Market Street as it heads through residential areas, with PA 168 splitting to the east in a commercial area. PA 551 continues north past more homes, turning east onto 5th Street as it forms the border between Darlington to the south and Darlington Township to the north. The route turns north onto Hollow Road and fully enters Darlington Township, heading into a mix of farms and woods with some residences. The road curves to the northwest as it heads through more rural areas.