The Vienna U-Bahn (German: U-Bahn Wien) is a rapid transit system serving Vienna, Austria. The five-line network consists of 83.1 kilometers (51.6 mi) of route, serving 109 stations. 459.8 million passengers rode the U-Bahn in 2019.

The modern-day U-Bahn opened on 25 February 1978, after test operations that began on 8 May 1976. Parts of two of the lines, designated U4 and U6, date back to the Stadtbahn ("city railway") system, which opened in 1898. Parts of the U2 and U6 lines began as subway tunnels built to accommodate earlier tram lines. Only the U1 and U3 were built wholly as new subway lines.

Lines are designated by a number and the prefix "U" (for U-Bahn) and identified on station signage and related literature by a colour. There are five lines; U1, U2, U3, U4 and U6. Since the late 1960s there have been numerous suggestions of routings for a line U5, but all these projects had been shelved until the construction of a new U5 was announced in early 2014. Stations are often named after streets, public spaces, or districts, and in some special cases after prominent buildings at or near the station. The policy of the Wiener Linien, however, states that they prefer not to name stations after buildings.^{[citation needed]}

Ticketing for the network is integrated under the Verkehrsverbund Ost-Region (VOR) along with other means of public transport in Vienna, including trams and buses. Local tickets are valid on S-Bahn suburban rail services and other train services but those are operated by the state railway operator, ÖBB. Tickets are not valid on bus services operated by Vienna Airport Lines or the City Airport Train express train.

With the September 2017 opening of the 4.6-kilometer (2.9 mi), five-station extension of the U1 line, the five-line U-Bahn network consists of 83.1 kilometers (51.6 mi) of route, serving 109 stations. Further extensions of the Vienna U-Bahn are scheduled to be completed in 2026-2032, finally creating the missing line U5. Upon completion of the U5 and U2 projects, there will then be a network that is 90 kilometers (56 mi) long with 116 stations. Some plans have been proposed for the system beyond 2032, when the U2/U5 project is completed, although such plans are currently unfunded.

U-Bahn services run between 05:00 and around 01:00 at intervals between two and five minutes during the day and up to eight minutes after 20:00. Since 4 September 2010, there has been 24-hour service operating at 15-minute intervals on Friday and Saturday evenings, and on evenings prior to a public holiday. The 24-hour U-Bahn is supplemented on these nights by the Vienna NightLine bus service.

The stretch between Heiligenstadt and Hütteldorf, covering modern day U4, used to be part of the metropolitan railway (Stadtbahn). The stretch between Längenfeldgasse and Spittelau, covering most of U6, also used to be a part, although Spittelau wasn't a station yet. The stretch between Karlsplatz and Schottenring (covering some of U2, and in the future some of it will cover a section of U5) was part of an underground tram network (U-Straßenbahn).

Planning for an underground railway in Vienna can be traced back to the 1840s. Since then, there have been numerous plans and concessions to build such a project, making Vienna the city with the most subway planning.^{[citation needed]}