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 construction 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.

An ocean liner is a passenger ship primarily used as a form of transportation across seas or oceans. Liners may also carry cargo or mail, and may sometimes be used for other purposes (such as for pleasure cruises or as hospital ships).Cargo vessels running to a schedule are sometimes called liners. The category does not include ferries or other vessels engaged in short-sea trading, nor dedicated cruise ships where the voyage itself, and not transportation, is the prime purpose of the trip. Nor does it include tramp steamers, even those equipped to handle limited numbers of passengers. Some shipping companies refer to themselves as "lines" and their container ships, which often operate over set routes according to established schedules, as "liners".

Ocean liners are usually strongly built with a high freeboard to withstand rough seas and adverse conditions encountered in the open ocean. Additionally, they are often designed with thicker hull plating than is found on cruise ships, and have large capacities for fuel, food and other consumables on long voyages.The first ocean liners were built in the mid-19th century. Technological innovations such as the steam engine and steel hull allowed larger and faster liners to be built, giving rise to a competition between world powers of the time, especially between the United Kingdom and Germany. Once the dominant form of travel between continents, ocean liners were rendered largely obsolete by the emergence of long-distance aircraft after World War II. Advances in automobile and railway technology also played a role. After Queen Elizabeth 2 was retired in 2008, the only ship still in service as an ocean liner is RMS Queen Mary 2.