The Green Line is a semi-metro system run by the Massachusetts Bay Transportation Authority (MBTA) in the Boston, Massachusetts, metropolitan area. It is the oldest MBTA subway line, and with tunnel sections dating from 1897, the oldest subway in North America. It runs underground through downtown Boston, and on the surface into inner suburbs via six branches on radial boulevards and grade-separated alignments. With an average daily weekday ridership of 137,700 in 2019, it is the third-most heavily used light rail system in the country. The line was assigned the green color in 1967 during a systemwide rebranding because several branches pass through sections of the Emerald Necklace of Boston.

Semi-metro is a form of public rail transport in which trams run partly on a conflict-free track, by using tunnels and/or viaducts. These stretches of track are designed to function like a regular metro or rapid transit line. Semi-metro lines run with tram cars because they are usually developed from an existing tram network.