Route 21 is a bus route operated by the Maryland Transit Administration in Baltimore. The line currently runs from the Mondawmin Metro Subway Station to Fells Point, serving the corridors of Gilmor Street, Preston/Biddle Streets, and Caroline Street, and the communities of Sandtown-Winchester, , and Butcher's Hill. The bus route is the successor to the 21 Preston Street–Caroline Street and Dolphin Street streetcar lines.