Pennsylvania Route 478 (designated by the Pennsylvania Department of Transportation as SR 478) is a 2.20-mile (3.54 km) long state highway located in Clarion and Venango Counties. The highway begins at an intersection with Pennsylvania Route 38 and Pennsylvania Route 208 northeast of Emlenton. The highway serves the westbound interchange for Exit 45 and terminates at Pennsylvania Route 58 in St. Petersburg.

The alignment of Route 478 began as part of the former alignment of Pennsylvania Route 238 until 1936, when Route 378 was designated on the alignment. In 1961, to avoid duplication with the newly numbered Interstate 378 in Bethlehem, the route was redesignated as Route 478.

Route 478 begins at an intersection with Pennsylvania Route 38 and its concurrency with Pennsylvania Route 208 northeast of Emlenton. The highway progressed to the southeast, crossing into Clarion County. There, Route 478 enters a partial interchange with Interstate 80, intersecting with the westbound ramps for Exit 45. After Interstate 80, Route 478 turns from its southward progression to the southeast, crossing through dense forestry. The highway soon turns back to the south, paralleling Freeman Road and entering the community of St. Petersburg. In St. Petersburg, Route 478 ecomes known as Salem Avenue, progressing southeast through residential homes and terminating at an intersection with Pennsylvania Route 58 (Main Street).

The alignment of Route 478 began in the 1928 numbering of state highways as an unnumbered state highway. However, the next year, the alignment was designated as part of Pennsylvania Route 238, which ran from Saint Petersburg to Shippensville. This lasted only seven years, when Route 238 was realigned to terminate in Mariasville. In 1936, the alignment was replaced with Route 378, which remained intact as its alignment designation until 1961, when Interstate 378 was designated for a freeway in Bethlehem. To avoid duplication, Route 378 was replaced with Route 478, a designation that had been unused until that point. The alignment has remained intact since.