Pennsylvania Route 158 (PA 158) is located in western Pennsylvania, running from PA Route 18 3 miles (4.8 km) south of New Wilmington in Lawrence County, to Mercer in Mercer County.

Traveling north from PA Route 18, PA 158 heads into the borough of New Wilmington, where it passes Westminster College and intersects with PA Route 208. The two routes briefly overlap eastbound; at the intersection with PA 956, the concurrency ends, and PA 158 continues northward. As it leaves the borough, it enters Mercer County and winds northeasterly through rural countryside nearly 6 miles (9.7 km) before it passes over I-80. Approximately 3 miles (4.8 km) later, the route enters the borough of Mercer, where it meets the eastern terminus of PA Route 318 just one block before its own northern terminus at the US 62/PA Route 258 concurrency.