The Sutton Underground Line was a proposed extension to the London Underground network in Sutton, South London that ultimately never materialised. Although Sutton does not have a direct Tube station today, historical plans indicate that, had certain factors played out differently, the area could have been connected to both the District line and the Northern line. The proposal was part of broader efforts in the early 20th century to expand London's Underground network and reduce the dominance of other railway companies.