Smith County (standard abbreviation: SM) is a county located in the U.S. state of Kansas. As of the 2010 census, the population was 3,853. Its county seat is Smith Center. The county is named in memory of Maj. J. Nelson Smith, who was part of the 2nd Colorado Cavalry, killed in action at the Battle of Westport on October 21, 1864.The geographic center of the contiguous United States is located within the county, near Lebanon.