There are 12 US Post Offices in Jefferson County serving a total of 77,712 residents equating to 6,476 residents per post office. It's estimated that approximately 124,037 packages are handled across the 12 county post offices annually. 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. 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. The county is bisected by the Arkansas River, which was critical to its development and long the chief transportation byway.