Jefferson County, Arkansas is a county located in the U.S. state of Arkansas. As of the 2010 census, the population was 77,435. Its county seat and largest city is Pine Bluff. Jefferson County is Arkansas's 21st county, formed on November 2, 1829, from portions of Arkansas and Pulaski counties, and named for Thomas Jefferson, third President of the United States. Jefferson County is included in the Pine Bluff, Arkansas Metropolitan Statistical Area.