Route 86 is a local bus route in Boston, Cambridge, and Somerville, Massachusetts, operated by the Massachusetts Bay Transportation Authority (MBTA) as part of MBTA bus service. It operates on a circumferential route between Sullivan Square station and Reservoir station via Union Square, Somerville, Harvard Square, and Brighton Center. In 2018, it had the 18th-highest weekday ridership on the system, though it ranked 37th by number of weekday trips. A 2018–19 MBTA review of its bus system found that route 86 had infrequent and unreliable service, including irregular scheduled headways, despite its high ridership and significance as a crosstown connecting route.

The Massachusetts Bay Transportation Authority (MBTA) operates 152 bus routes in the Greater Boston area. The MBTA has a policy objective to provide transit service within walking distance for all residents living in areas with population densities greater than 5,000 inhabitants per square mile (1,900/km2) within the MBTA's service district. Much of this service is provided by bus. In 2023, the system had a ridership of 91,459,700, or about 303,300 per weekday as of the fourth quarter of 2023.