Key bus routes of the Massachusetts Bay Transportation Authority (MBTA) system were the 15 routes that had high ridership and higher frequency standards than other bus lines, according to the 2004 MBTA Service Policy. Together, they accounted for roughly 40% of the MBTA's total bus ridership. These key bus routes ensured basic geographic coverage with frequent service in the densest areas of Boston, and connected to other MBTA services to give access to other areas throughout the region.

In recognition of their function as part of the backbone MBTA service, the key bus routes were added to newer basic route maps installed in subway stations and other public locations. These schematic route maps show the rail rapid transit routes, bus rapid transit routes, commuter rail services, and key bus routes. The key routes had been treated as a distinct category for the purpose of service improvement, such as trial runs of late-night service, and due to the high volume of passenger traffic they carry, both individual routes and the category as a whole have been the subjects of urban planning and transportation engineering studies. As part of the implementation of the MBTA's Bus Network Redesign program in 2024, the key bus route terminology is being phased out and replaced by a general high-frequency route network.

In November 2006, the MBTA launched a concerted effort to improve service quality on key bus routes. The 2008 Service Plan recommended improvements for various lines, including upgrading the 31 bus to key route standards. A second round of upgrades, entitled the Key Routes Improvement Project and costing $10 million in all, was supported by the American Recovery and Reinvestment Act of 2009. The Silver Line services were originally considered part of this program, but were later split off into a separate enhancement project. The initial Key Routes Improvement Project was concluded in 2013, with some planned upgrades not being made. Due to these changes, the reliability of service (as measured by adherence to posted service frequencies) has been found to be greater on the Key Routes than on others, with 75% versus 61% respectively in June 2017.

A 2012 statistical analysis found that proximity to key bus routes was inversely correlated to taxicab trip generation, whereas taxicab trip generation actually increased with proximity to bus routes overall, suggesting that only the routes with highest frequency were able to compete with taxi service among customers with the option to choose.

In April 2014, extended late-night service on the key bus routes and rapid transit routes was announced, to be operated on a one-year trial basis with service continuation depending on late-night ridership and on corporate sponsorship. As of April 2015^[update], late-night service was cut back from 2:30am to 2:00am, and extended hours were dropped from 5 of the 15 key bus routes. Late-night service was discontinued altogether in March 2016.

The category of key bus routes figured into Boston mayor Marty Walsh's "Go Boston 2030" initiative, which included the goal of having "every Boston household" within a 10-minute walk from a T station or a bus stop on a key route. In addition, a new key route was proposed to connect the Longwood Medical and Academic Area with JFK/UMass station via Roxbury.

In May 2022, the MBTA released a draft plan for a bus network redesign, which included potential changes to most of the key routes. A number of other routes would be upgraded to key bus route frequency, forming a high-frequency bus network complementing the rapid transit network. A revised plan was released in November 2022.

The MBTA's Service Delivery Policy uses five criteria when determining if a route is part of the Key Bus Routes program.