Barton County is a county located in the southwestern part of the U.S. state of Missouri. As of the 2020 census, the population was 11,637. Its county seat is Lamar. The county was organized in 1855 and named after U.S. Senator David Barton from Missouri.

Lamar is a city in and the county seat of Barton County, Missouri, United States. As of the 2020 census, the city population was 4,266. It is known as the birthplace of Harry S. Truman, the 33rd president of the United States.