Arnold is the largest city in Jefferson County, Missouri, United States. The population was 20,808 at the 2010 census.
The first European settler in Arnold was Jean Baptiste Gamache, who operated a ferry boat across the Meramec River in exchange for 1050 arpents of land granted by the King of Spain. This ferry was on the King's Trace or El Camino Real, from St. Louis to Ste. Genevieve, Missouri.
Arnold was a focal point over the eminent domain issue in Missouri. In January 2004, the City of Arnold announced that THF Realty had approached them regarding developing a section of Arnold known as the Triangle, an area bordered by Route 141, Interstate 55 and Church Road in the city limits. The city voted in favor of the Triangle Development project proceeding on September 16, 2005.
According to an agreement with THF, Arnold would acquire the properties in the triangle and would be reimbursed its costs by THF. Arnold offered the property owners a buy-out, and most accepted.