Knox is a borough in Clarion County, Pennsylvania, United States. The population was 1,146 at the 2010 census.
Knox is located northwest of the center of Clarion County at 41°14′7″N 79°32′10″W (41.235317, -79.536123). Pennsylvania Route 208 passes through the center of the borough, leading east 4 miles (6 km) to U.S. Route 322 at Shippenville and west 11 miles (18 km) to Emlenton. Pennsylvania Route 338 runs through the east side of Knox as a two-lane bypass; it leads north 4 miles (6 km) to US 322 and south four miles to Interstate 80. Clarion, the county seat, is 9 miles (14 km) to the east via Shippenville.
According to the United States Census Bureau, Knox has a total area of 0.57 square miles (1.47 km2), of which 0.004 square miles (0.01 km2), or 0.68%, is water.
As of the census of 2000, there were 1,176 people, 528 households, and 337 families residing in the borough. The population density was 2,056.0 people per square mile (796.6/km²). There were 570 housing units at an average density of 996.5 per square mile (386.1/km²). The racial makeup of the borough was 98.98% White, 0.60% Native American, 0.17% Asian, and 0.26% from two or more races. Hispanic or Latino of any race were 0.85% of the population.