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 route turns north onto Hollow Road and fully enters Darlington Township, heading into a mix of farms and woods with some residences.

[2][3] PA 551 enters the borough of Enon Valley in Lawrence County and becomes Main Street, heading into residential areas and crossing Norfolk Southern's Fort Wayne Line.

PA 551 crosses Norfolk Southern's Youngstown Line and becomes Erie Street, heading into the residential community of Edinburg.

Here, the route turns west onto Jackson Street and passes more homes before heading through farm fields and curving northwest.

PA 551 comes to an intersection with US 224 and turns northeast to join that route on West State Street.

The road heads through wooded areas and crosses the Mahoning River and CSX's New Castle Subdivision railroad line.

The route continues through a mix of agricultural areas and woodland, curving north as it comes to its northern terminus at another intersection with PA 18.