Montreal's Underground City (officially RÉSO or La Ville Souterraine in French) is the set of interconnected complexes (both above and below ground) in and around Downtown Montreal, Quebec, Canada.

Not all portions of the indoor city (ville intérieure) are underground. The connections are considered tunnels architecturally and technically, but they are air-conditioned and have lighting as good as any building's livable space does. Many tunnels are large enough to have shops on both sides of the passage. With over 32 km (20 mi) of tunnels spread over more than 12 km (4.6 sq mi), connected areas include shopping malls, apartment buildings, hotels, condominiums, banks, offices, museums, universities, seven Metro stations, two commuter train stations, a regional bus terminal, and the Bell Centre amphitheatre and arena. There are more than 120 exterior access points to the underground city. Each access point is an entry point to one of 60 residential or commercial complexes comprising 3.6 km (1.4 sq mi) of floor space, including 80% of all office space and 35% of all commercial space in downtown Montreal. In winter, some 500,000 people use the underground city every day.