Sullivan Square is a traffic circle located in the Charlestown neighborhood of Boston, Massachusetts. The MBTA's Sullivan Square station is located to the west of the square. Adjacent to the East Somerville area of Somerville, Sullivan Square is named after James Sullivan, an early 19th-century Governor of Massachusetts. In the early 2000s, the Sullivan Square Overpass was dismantled, which left a stub approach to Route 99.
Sullivan Square stands on what was once a narrow neck of land referred to as the Charlestown Neck, an area that was originally a thin strip of land connecting the Charlestown Peninsula with present-day East Somerville. Being in a narrow place between larger land masses made Sullivan Square a place where transportation routes naturally converged, and various transportation facilities developed here over the years.
Of particular note was the construction of the Middlesex Canal which traveled 27 miles from Lowell to terminate at the Mill Pond in Charlestown, passing directly through where the Sullivan Square traffic circle stands today. Completed in 1803, the canal was considered a major engineering feat at its time. However, the Boston and Lowell Railroad, completed in 1835, captured much of the freight business, and the canal ceased operation by 1853. The Boston and Lowell and the Fitchburg, the earliest railroads to pass through Somerville, did not come through this area of the city, but in 1842 the Boston and Maine Corporation opened a station near Sullivan Square. This led to the construction of a residential enclave for commuters to Boston. New streets were laid out, such as Mt. Vernon Street and Mt. Pleasant Street, and small lots were plotted out along them.
The next transportation revolution to make its way through Sullivan Square was the streetcar. Horsecars connecting South Medford, and Somerville to Charlestown and Boston started running in 1858, just two years after the first horsecar in Massachusetts connected Cambridge and Boston. Horsecar travel continued to grow until the early 1890s, when electrically powered street railways became popular. Sullivan Square came to be the location of a large carhouse, and in 1901 a grand new station was built to serve both streetcars and the Boston Elevated Railway, the new rapid transit line connecting Sullivan Square and Dudley Square by way of downtown Boston.
The original elevated station was considered the crown jewel of the "El." Surface streetcars ran up to the level of the elevated stations, allowing cross-platform transfer underneath an expansive trainshed. The station was designed as a major transfer point, with many streetcar lines that had gone downtown truncated to Sullivan. However, the station quickly reached capacity, and an extension to Everett opened in 1919. However, beginning in 1963 the Everett terminus was closed nights and Sundays, and the truncated routes (now all buses) were extended back to Sullivan Square at those times.
Over time, deferred maintenance on the Sullivan Square station took its toll, and when the Charlestown Elevated came down in the 1970s, the once-magnificent transportation hub was considered by some to be beyond saving. The current Sullivan Square station was opened as part of the Haymarket North Extension project on April 7, 1975, on top of now-defunct Boston and Maine Railroad right-of-way. Shortly thereafter, a mysterious fire broke out in the old station, which was subsequently razed and repurposed as an MBTA service vehicle parking lot. Simultaneously, double-deck Interstate 93 was built over the station 
The Encore Boston Harbor is funding $11 million of short-term improvements Theto the rotary and surrounding roads, adding a bus bypass, adding some new roads to allow some cars to bypass the rotary, adding traffic lights, and improving bicycle and pedestrian infrastructure, and improvements to the MBTA busway and parking lot. These started in 2018 and are expected to be in place for the casino opening in 2019. A longer-term plan which eliminates the rotary entirely and establishes a local street grid is planned for 2021-2025, with $25 million from the casino and about $150 million from the Boston Region Metropolitan Planning Organization.
- "Boston's Neighborhoods: Charlestown". Boston Redevelopment Authority (BRA). 2010. Retrieved August 17, 2010.
- "James Sullivan (1744-1808)". mass.gov. James Sullivan (1744-1808). 2013. Retrieved 6 December 2013.
- Daniel, Mac (May 24, 2002). "Bad to worse? Some say razing overpass will heighten traffic woes". Boston Globe.
- cf. Haskell, Albert L., "Haskell's Historical Guide Book of Somerville, Massachusetts", section on "Somerville: Why So Named".
- "Sullivan Square: Part 1 of 3". Somerville Development Forum. Somerville Development Forum. Retrieved 6 December 2013.
- "Boston Elevated Railway Company, Main Line Elevated Structure (HAER)". NYCsubway.org. NYC Subway. Retrieved 6 December 2013.
- "Sullivan Square: Part 3 of 3". Somerville Development Forum. Somerville Development Forum. Retrieved 6 December 2013.
- Sullivan Square Lower Busway and Parking Lot Reconstruction
- With Wynn’s casino rising nearby, city has plans for Sullivan Square