RMS Queen Mary 2 (also referred to as the QM2) is a British transatlantic ocean liner. She has served as the flagship of Cunard Line since succeeding Queen Elizabeth 2 in 2004. As of 2022, Queen Mary 2 is the only ocean liner (as opposed to a cruise ship) still in service.The ship was officially named Queen Mary 2 by Queen Elizabeth II in 2004 after the first RMS Queen Mary of 1936. Queen Mary had in turn been named after Mary of Teck, consort of King George V. With the retirement of Queen Elizabeth 2 in 2008, Queen Mary 2 is the only transatlantic ocean liner in regular service between Southampton, England, and New York City, United States. The ship is also used for cruising, including an annual world cruise.She was designed by a team of British naval architects led by Stephen Payne, and was constructed in France by Chantiers de l'Atlantique. At the time of her construction, Queen Mary 2 was the longest, at 1,131.99 ft (345.03 m), and largest, with a gross tonnage of 148,528 GT, passenger ship ever built. She no longer holds these records after the construction of Royal Caribbean International's 154,407 GT Freedom of the Seas (a cruise ship) in April 2006, but remains the largest ocean liner ever built.

Queen Mary 2 was intended for regular scheduled crossings of the Atlantic Ocean; the final cost was approximately $300,000 per berth. The cost was increased by the high quality of materials; having been designed as an ocean liner, 40% more steel was required than for a standard cruise ship. Queen Mary 2 has a maximum speed of just over 30 knots (56 km/h; 35 mph) and a cruising speed of 26 knots (48 km/h; 30 mph), which is faster than a contemporary cruise ship. Instead of the common diesel-electric configuration, Queen Mary 2 uses integrated electric propulsion to achieve her top speed. Diesel engines, augmented by gas turbines, are used to generate electricity for electric motors for propulsion and for on-board use.

Queen Mary 2's facilities include fifteen restaurants and bars, five swimming pools, a casino, a ballroom, a theatre, and the first planetarium at sea.