The Glasgow Subway is a metro system in Glasgow, Scotland. Opened in 1896, it is the third oldest subway system in the world after the London Underground and the Budapest Metro. Originally a cable railway, the Subway was later electrified, but its one circular line has never been expanded. Officially called the Glasgow Underground between 1936 and 2003, it has reverted to its colloquial name of Subway. It remains one of only two underground metro-type systems in the UK outside London, the other being the Tyne and Wear Metro. A £40,000 study examining the feasibility of an expansion into the city’s south side is in progress.

The circular route is almost 6.5 miles (10.4 km) long and extends both north and south of the River Clyde. The tracks have the unusual narrow gauge of four feet (1219 mm), and a nominal tunnel diameter of 11 feet (3.35m), comparable to that of the deep-level lines of the London Underground (11 feet 8.25 inches or 3.56m), though the rolling stock is significantly smaller.