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 (Cleveland Circle) 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.