Clarion is a borough in and the county seat of Clarion County, Pennsylvania, United States. It is located north-northeast of Pittsburgh and is part of the Pittsburgh DMA. Clarion was settled in 1839 and incorporated in 1841. In the past, the surrounding area produced natural gas, oil, lumber and coal. The population was 2,004 in 1900, 2,864 in 1910, and 5,276 at the 2010 census. It is home to the annual Autumn Leaf Festival and Clarion University of Pennsylvania. The county courthouse was added to the National Register of Historic Places in 1979.