Jefferson County, Arkansas is a county located in the U.S. state of Arkansas in the area known as the Arkansas Delta that extends west of the Mississippi River. Jefferson County consists of five cities, two towns, and 20 townships. It is bisected by the Arkansas River, which was critical to its development and long the chief transportation byway. In 2020, Jefferson County's population was estimated at 67,260. The county seat and largest city is Pine Bluff.