Arnold, a census-designated place in Anne Arundel County, Maryland, United States, located just outside of the state's capital, Annapolis. It is located 18.78 miles south of Baltimore, and 29.97 miles east of Washington, D.C. The population was 23,106 at the 2010 census. Neighborhoods straddle College Parkway and Maryland Route 2. Arnold is located on the scenic Broadneck Peninsula. The ZIP code is 21012.