Pennsylvania Route 158 is an 11.9-mile-long (19.2 km) state highway located in western Pennsylvania, running from PA 18 south of New Wilmington in Lawrence County, to US 62 and PA 258 in Mercer in Mercer County.

Pennsylvania Route 18 is a major north–south highway in Western Pennsylvania whose southern terminus is at West Virginia Route 69 at the state line in Greene County near the village of Garrison, while the northern terminus is at PA 5 in Lake City. At a length of 205 miles (330 km), PA 18 is the only state route in Pennsylvania — north–south or east–west — to traverse the entire state. It also has the distinction of being the longest state route in Pennsylvania.