There are 13 US Post Offices in Sebastian County serving a total of 144,668 residents equating to 11,128 residents per post office. It's estimated that approximately 230,906 packages are handled across the 13 county post offices annually. Sebastian County is a county located in the U.S. state of Arkansas. As of the 2010 census, the population was 125,744, making it the fourth-most populous county in Arkansas. The county has two county seats, Greenwood and Fort Smith. Sebastian County is part of the Fort Smith, AR-OK Metropolitan Statistical Area.