PA 125 begins at an intersection with PA 443 in the borough of Pine Grove in Schuylkill County, heading northwest on two-lane undivided North Tulpehocken Street. The road passes homes and a few businesses before crossing into Pine Grove Township and becoming Tremont Road. Here, the route heads north into wooded areas with some development, following the Swatara Creek which is situated east of the road and passing through North Pine Grove. PA 125 begins to run a short distance to the east of I-81 as it passes through a forested gap of Second Mountain and crosses the creek. The road heads into Tremont Township and turns northeast as South Tremont Road, intersecting Molleystown Road, which continues northwest to interchange with I-81 a short distance to the west. In the vicinity of this intersection, the route is a divided highway. PA 125 becomes undivided again and runs through more forests parallel to the Swatara Creek, turning north-northwest as it passes through Echo Valley and crosses Sharp Mountain. The road curves to the northeast again before another turn to the north, crossing Swatara Creek. The route crosses a Reading Blue Mountain and Northern Railroad line prior to entering the borough of Tremont and heading northeast on Branch Street, passing through wooded areas of homes. PA 125 heads back into Tremont Township and comes to an intersection with US 209, turning southwest to form a concurrency with that route. The two routes head back into Tremont and merge onto East Main Street, heading into business areas. PA 125 splits from US 209 by heading northwest on Spring Street. The route heads past homes, turning to the west and passing a mix of residences and industrial establishments.