Clarksville is a city in Johnson County, Arkansas, United States. As of the 2010 census the population was 9,178, up from 7,719 in 2000. As of 2018, the estimated population was 9,743. The city is the county seat of Johnson County. It is nestled between the Arkansas River and the foothills of the Ozark Mountains, and Interstate 40 and US Highway 64 intersect within the city limits.