The Vienna U-Bahn (German: U-Bahn Wien), where U-Bahn is an abbreviation of the German term Untergrundbahn (English: underground railway), is one of the two rapid transit (metro) systems for Vienna, Austria. The second system is the Vienna S-Bahn. With the October 2013 opening of the 4.2 kilometers (2.6 mi), three-station extension of the U2 line, the five-line U-Bahn network consists of 78.5 kilometers (48.8 mi) of route, serving 98 stations. It is the backbone of one of the best performing public transport systems worldwide according to UITP (International Association of Public Transport) in June 2009. More than 1.3 million passengers rode the Vienna U-Bahn every day in 2009, and 567.6 million passengers used the U-Bahn in 2011, which declined to 428.8 million passengers in 2013. The network is undergoing expansion and rolling stock renewal. Since 1969, 200 million euros have been invested annually in the extension of the Vienna U-Bahn.

The modern U-Bahn opened on 25 February 1978 (after test operations began on 8 May 1976), but two of the lines extended and later designated as U-Bahn (U4, 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.