Arnold is the largest city in Jefferson County, Missouri, United States. The population was estimated at 20,603 in 2008. It was 19,965 in the 2000 census.
Arnold is located at 38°25′58″N 90°22′10″W / 38.43278°N 90.36944°W / 38.43278; -90.36944 (38.432753, -90.369393). According to the United States Census Bureau, the city has a total area of 11.6 square miles (30.0 km²), of which, 11.2 square miles (29.1 km²) of it is land and 0.3 square miles (0.9 km²) of it (2.85%) is water.
Arnold was incorporated in 1972, and is named after George Arnold, the city's first postmaster. Four communities merged with Arnold, and their old names still have some currency: Beck (named after local farmer Emmanual Beck), Flamm City, Maxville (named after Max Stengel, first business owner), and Ten Brook (named after landowner John Ten Brook).