Elkridge is an unincorporated community and census-designated place (CDP) in Howard County, Maryland, United States. The population was 15,593 at the 2010 census. Founded early in the 18th century, Elkridge is adjacent to two other counties, Anne Arundel and Baltimore.

The Thomas Viaduct spans the Patapsco River and Patapsco Valley between Relay, Maryland and Elkridge, Maryland, USA. It was commissioned by the Baltimore and Ohio Railroad (B&O); built between July 4, 1833, and July 4, 1835; and named for Philip E. Thomas, the company's first president. Some claim it to be the world's oldest multiple arched stone railroad bridge. However, the Sankey Viaduct on the Liverpool and Manchester Railway was opened in 1830 and finally completed in 1833.