Route 203 is a 5.34-mile-long (8.59 km) east-west state highway located wholly within the city of Boston, Massachusetts.

Route 203 is poorly signed, but runs along part of the Arborway, Morton Street and Gallivan Boulevard, all parkways formerly part of the Metropolitan District Commission system of parks and roads.

Prior to the creation of the Massachusetts Department of Transportation (MassDOT) in 2009, the route was owned and maintained by the Department of Conservation and Recreation (DCR, previously the Metropolitan District Commission).

William Casey Highway overpass in Jamaica Plain, Morton Street in Mattapan and Gallivan Boulevard in Dorchester were transferred to MassDOT, while the Arborway continues to be maintained under the DCR.

[2][3] Route 203 was designated in the early 1970s as part of a large Boston-area renumbering.