Pennsylvania Route 18 (PA 18) is a major north–south highway in Western Pennsylvania whose southern terminus is at the West Virginia state line, where it continues as WV 69, while the northern terminus is at PA 5 in Lake City.

Traveling northward from West Virginia Route 69 at the West Virginia state line, Route 18 winds through rural Greene County, passing through the villages of Garrison, New Freeport, Nettle Hill, White Cottage, Woodruff, and Holbrook, before making its first junction with another state highway, PA Route 21, just west of the village of Rogersville and over 15 miles (24 km) from the state line.

Here the two routes overlap for nearly 6 miles (9.7 km), winding east-northeast through Rogersville and the village of East View, crossing the South Fork of Ten Mile Creek numerous times before reaching the village of West Waynesburg.

At this point, Route 18 leaves the concurrency and turns northwest, traveling 12 miles (19 km) to the county line, following along Browns Creek, and passing through the villages of Rees Mill, Sycamore, and Nineveh.

US 40 then leaves the concurrency, and nearly a mile later, Route 18 interchanges with I-70, connecting via city streets to the entrance and exit ramps.

Passing through nearly 5 miles (8.0 km) of state park land, Route 18 eventually meets US 30 in the village of Harshaville.

As Route 18 approaches the Ohio River, it turns east-northeast before its first interchange with I-376 about 2 miles (3.2 km) west of the borough of Monaca.

The concurrency enters the borough of New Brighton, where Route 18 then leaves the concurrency over 2 miles (3.2 km) later, crosses the Beaver River, enters the city of Beaver Falls, and continues northward almost 2 miles (3.2 km).

The route then passes through the borough of Homewood about 1.5 miles (2.4 km) later, and re-enters Big Beaver, where it interchanges with I-76 and The Pennsylvania Turnpike.

The two join and turn east, then northeast through the village of Mahoningtown, situated between the Mahoning and Shenango rivers.

The concurrency then passes under US 422 about 0.8 miles (1.3 km) later before turning eastward and crossing the Shenango River.

From here, PA 18 heads into a rural area of northern Lawrence County that is home to an Amish community.

After entering Crawford County, Route 18 then passes through the village of Adamsville about 2 miles (3.2 km) north of the county line, and, over 2.5 miles (4.0 km) later, enters the village of Hartstown, where it joins US 322, heading east-northeast.

The bridge along Mahoning Avenue was prohibited to trucks over 30 tons, necessitating the route.

In 2018, the bridge underwent rehabilitation, including the replacement of bearings, and substructure and steel repairs, among other fixes.