Lamar is a city in Barton County, Missouri, United States. The population was 4,474 at the 2011 census. It is the county seat of Barton County. Lamar is well known as the birthplace of President Harry S. Truman.
Lamar is located at 37°29′37″N 94°16′20″W / 37.49361°N 94.27222°W / 37.49361; -94.27222 (37.493676, -94.272111). According to the United States Census Bureau, the city has a total area of 4.1 square miles (10.8 km²), of which, 3.8 square miles (9.9 km²) of it is land and 0.3 square miles (0.8 km²) of it (7.25%) is water.
As of the census of 2000, there were 4,425 people, 1,835 households, and 1,154 families residing in the city. The population density was 1,154.5 people per square mile (446.1/km²). There were 1,995 housing units at an average density of 520.5 per square mile (201.1/km²). The racial makeup of the city was 96.61% White, 0.18% African American, 0.61% Native American, 0.34% Asian, 0.14% Pacific Islander, 0.34% from other races, and 1.79% from two or more races. Hispanic or Latino of any race were 1.40% of the population.