Route 203 was formed in the early 1970s as part of a large Boston-area renumbering. Most of the route had been part of Route 3, which came south along The Jamaicaway with U.S. Route 1 and split to the east along present Route 203. Route 3 turned south at Granite Avenue to join the Southeast Expressway in Milton, and Route 3A began where Route 3 turned. With the renumbering, Route 3 was kept on the Southeast Expressway into downtown, Route 3A was truncated to its current end, and the former Route 3 and Route 3A became Route 203. While the former routes had been signed north–south, the new route was signed east–west.
The only other change has been the removal of U.S. Route 1 from the west end of Route 203; it no longer ends at a numbered route at its west end.