Belton is a city in northwestern Cass County, Missouri, United States. The population was 23,953 at the 2020 census. It is a part of the Kansas …Belton is a city in northwestern Cass County, Missouri, United States. The population was 23,953 at the 2020 census. It is a part of the Kansas City Metropolitan Area. Belton was platted in 1871. The city was likely named for surveyor Capt. Marcus Lindsey Belt. A post office called Belton has been in operation since 1872.