The Dewey Square Tunnel in Boston, Massachusetts, is part of Interstate 93, running under the heart of the city's financial district. Constructed in 1959, it was part of the original Central Artery project of the 1950s.

Known to locals as the South Station Tunnel (due to its proximity to the city's main train station), the Dewey Square Tunnel is of cut-and-cover design, and originally was six lanes wide (three in each direction), with no breakdown lanes. The Central Artery, elevated for most of its length, was so reviled because it cut off views of and access to Boston's historic waterfront. It was derisively called "the other Green Monster" (not to be confused with the famous left-field wall at nearby Fenway Park with the same nickname). Because of public outcry, it was decided that the final section of the artery was to be put underground.

The Dewey Square Tunnel is receiving new life as part of the massive Central Artery/Tunnel (Big Dig) project. It is being rehabilitated and will serve as exclusive southbound lanes of I-93, connecting at its current northern portal with the new underground southbound lanes of the new artery. The refurbished tunnel, which will have 5 travel lanes, is now expected to be fully open by March 2005.

When the tunnel was built, part of the unused upper level of the MBTA Red Line tunnel was removed; this level is now used for storage, and further northwest as the Winter Street Concourse, a walkway inside fare control between Park Street and Downtown Crossing stations.