The Blue Line is a rapid transit line in the Greater Boston metropolitan area, one of four rapid transit lines operated by the Massachusetts Bay Transportation Authority (MBTA). It runs from Bowdoin station in downtown Boston under Boston Harbor to East Boston and Revere on the inner North Shore, where it terminates at Wonderland. The stop at Airport Station, by way of a free shuttle bus, is one of two rapid transit connections to Logan International Airport. In 1967, during a systemwide rebranding, the line was assigned the blue color because it passes under the Boston Harbor. With an end-to-end travel time of less than twenty minutes, the Blue Line is the shortest of Boston's heavy-rail lines and the only line to have both third rail and overhead catenary sections.

The Massachusetts Bay Transportation Authority (MBTA) operates rapid transit, light rail, and bus rapid transit services in the Boston metropolitan area, collectively referred to as the rapid transit, subway, or the T system.