The C branch, also called the Beacon Street Line or Cleveland Circle Line, is one of four branches of the Massachusetts Bay Transportation Authority's Green Line light rail system in the Boston, Massachusetts metropolitan area. The line begins at Cleveland Circle in the Brighton neighborhood of Boston and runs on the surface through Brookline along the median of Beacon Street. Reentering Boston, the line goes underground through the St. Mary's Street incline and joins the B and D branches at Kenmore. Trains run through the Boylston Street subway to Copley where the E branch joins, then continue through the Tremont Street subway to downtown Boston. The C branch has terminated at Government Center station since October 2021.

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.