Clarion County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 37,241. Its county seat is Clarion. The county was formed on March 11, 1839, from parts of Venango and Armstrong counties. Clarion County is entirely defined as part of the Pittsburgh media market. The county is part of the North Central Pennsylvania region of the state.

The Clarion River is a tributary of the Allegheny River, approximately 110 mi (177 km) long, in west central Pennsylvania in the United States. It drains a rugged area of the Allegheny Plateau in the Ohio River watershed, flowing through narrow serpentine valleys and hardwood forests.