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. The city government is consolidated with City Township. Because the city governance and township governance have different roles. circa 2002 the U.S. Census Bureau counted them as separate governments for census purposes.