thumb|right|Geographic map of MBTA bus service in 2016
The Massachusetts Bay Transportation Authority bus division operates bus routes in the Boston, Massachusetts metropolitan area. All routes connect to MBTA subway, MBTA Commuter Rail, and/or other MBTA bus services. Many routes are descendants of the streetcar routes of the Boston Elevated Railway, or of suburban companies including the Eastern Massachusetts Street Railway and Middlesex and Boston Street Railway. The MBTA directly operates routes, while private companies operate routes under contract. Three additional suburban operations are partially funded by the MBTA.
Most buses charge local bus fare ($1.70 one-way as of 2024). The Silver Line waterfront services (SL1, SL2, SL3, and SLW) charge the standard subway fare ($2.40 one-way as of 2024). Express buses have a local portion within a community (which charge the local bus fare), and an express portion that takes a highway to or from downtown Boston (which charges a higher fare of $4.25 as of 2025).
A number of routes were temporarily suspended or placed on modified routings during the COVID-19 pandemic; overall service levels were restored to pre-COVID levels in 2021, though some routes were not restored. Since 2018, the MBTA has been planning a major bus network overhaul, with implementation expected to be complete in 2028. The first changes took place in December 2024.
Silver Line
thumb|right|SL2 bus at South Station
The Silver Line is a six-route bus rapid transit system marketed as rapid transit. It is divided into two branches: Waterfront service (SL1, SL2, SL3, and the rush-hour SLW shuttle) that runs through the South Boston Transitway tunnel, and Washington Street service (SL4 and SL5) that runs on the surface via Washington Street. The Waterfront service costs the same as a subway fare, while the Washington Street service costs a regular bus fare. All Silver Line routes use articulated buses; the Waterfront routes use hybrid buses with extended battery range for electric operation in the tunnel.
The SL5 route was created in 2002 as a replacement for the Washington Street Elevated. The Shuttle route began operations in 2004, followed by the SL2 and SL3 (former) in 2004 and SL1 in 2005 when dual-mode buses became available. The SL4 was introduced in 2009 as a replacement for the canceled Phase III tunnel. A new SL3 route to Chelsea opened in 2018. The BERy folded into the Metropolitan Transit Authority in 1947, and the M.T.A. in turn was reorganized as the MBTA in 1964. Despite some changes, including minor routes being merged into trunk routes, the core service network has remained roughly intact since the BERy eta. New routes have been added during the M.T.A. and MBTA eras.
Seven routes – , , , , , , and – serve more distant western suburbs including Bedford, Waltham, Lexington, and Needham. They are descendants of routes acquired from the Middlesex and Boston Street Railway in 1972, which were subsequently renumbered using previously discontinued designations. As part of the implementation of the MBTA's Bus Network Redesign program beginning in 2024, the key bus route terminology is being phased out and replaced by a larger frequent route network. Routes 28 and 39 use 60-foot articulated buses.
Four early morning round trips are run between outlying stations and , each running over portions of several local routes. Although intended primarily for station agents, they are open to all passengers. The trips are internally numbered 191–194, but are shown in timetables as variants of local routes.