PA 58 travels eastward from the Ohiostate line in northwestern Mercer County just over to the borough of Jamestown, where, after passing through a low-clearance tunnel beneath an abandoned railroad right-of-way, it intersects with US 322. The two routes join together and continue eastward nearly where US 322 leaves northward. PA 58 continues eastward, then turns southeasterly, and about later, joins PA Route 18 just north of the Greenville borough line. The concurrency continues into the borough to its junction with PA Route 358. The three routes form a concurrency and head westward – this segment is what is known as a wrong way concurrency, because PA 58 is signed east at the same time that Route 358 is signed west. The concurrency lasts only a quarter mile, and then PA 58 leaves the concurrency, heading south-southeast nearly to the borough of Mercer. Here, the route first meets the US 19/US 62 concurrency. The three routes form a concurrency south to the intersection of PA Route 258. At this complex intersection, the concurrency ends, US 62 continues westward, concurrent with Route 258, US 19 continues south, and Routes 58 and 258 join to the east. Also at this point, the PA 58/PA 258 concurrency is split – eastbound follows S. Diamond St., and westbound follows N. Diamond St. in the middle of town. At the east end of the “square,” the concurrency ends as Route 258 heads southward and PA 58 continues east, then later turns southeast. It crosses both I-80 and I-79 near the interchange of the two interstates, just over from Mercer, and enters the borough of Grove City about later. In Grove City, the route intersects with the southern terminus of the PA Route 173/PA Route 208 concurrency, and overlaps with Route 173, continuing southeast. About later, the concurrency ends; Route 173 continues south, and PA 58 continues southeast just over to the Butler county line after spending 39 miles in Mercer County. A mile after entering Butler County, PA 58 intersects with PA Route 8 in the borough of Harrisville. The route then travels nearly to intersect with PA Route 308. The next major intersection is about later at PA Route 38 in the borough of Eau Claire. The route then travels nearly before crossing the next county line. The road spends about 18.5 miles in Butler County, serving as the main thoroughfare across the rural northern portion of the county.
Armstrong and Clarion counties
PA 58 has a very short stay in Armstrong County, passing through its extreme northwestern tip. Just from the county line, the route intersects with PA Route 268, it then crosses Allegheny River on the Foxburg Bridge, leaving the county just about later. Turning northeasterly, the route passes through the borough of Foxburg upon entering Clarion County, and then heads northeast to enter the borough of St. Petersburg a little over later. Here, PA 58 intersects with the eastern terminus of PA Route 478, and then about later, it intersects with the western terminus of PA Route 338 at the village of Alum Rock. The route then winds southeastward to intersect with the eastern terminus of PA Route 368 just east of the borough of Callensburg. PA 58 then travels east-southeast nearly to its terminus at PA Route 68 in the borough of Sligo The road spends 13 miles in Clarion County.
History
The route was originally signed in 1927 from New Wilmington to Greenville on the current PA 18 alignment. The route was moved in 1928 to the Mercer-to-Greenville alignment. Connecting PA 458 was decommissioned and reassigned as PA 58 in 1946, extending the route from its previous western terminus at US 322 in Jamestown to its current location at the Ohio state line. In 1967 the eastern terminus was moved from PA 8 in Harrisville to its current location.
