RMS Queen Mary 2 (QM2) is a British ocean liner. She has served as the flagship of the Cunard Line since April 2004, and as of 2025, is the only active, purpose-built ocean liner still in service. Queen Mary 2 sails regular transatlantic crossings between Southampton and New York City, in addition to short cruises and an annual world voyage.

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 some 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 for a cruise liner 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 (the largest on a current ship), a theatre, a library, a gym, an art gallery, a golf simulator, and the first planetarium at sea.

Queen Mary 2 is the flagship of Cunard Line. She was constructed to replace the aging Queen Elizabeth 2, which was the Cunard flagship from 1969 to 2004 and the last major ocean liner built before Queen Mary 2. Queen Mary 2 had the Royal Mail Ship (RMS) prefix conferred on her by the Royal Mail when she entered service in 2004, as a gesture to Cunard's history.

Queen Mary 2 is not a steamship like many of her predecessors, but is powered primarily by four diesel engines, with two additional gas turbines providing extra power when required; this integrated electric propulsion configuration is used to produce the power to drive her four electric propulsion pods as well as the ship's hotel services. The spaces for these prime movers are also split, and controls are also backed up, with the intention of preventing a single failure from disabling the ship.

Like her predecessor Queen Elizabeth 2 she is built for crossing the Atlantic Ocean, and is also regularly used for cruising. In the winter season she cruises from New York to the Caribbean. Queen Mary 2's 30-knot (56 km/h; 35 mph) open ocean speed sets the ship apart from cruise ships, such as MS Oasis of the Seas, which has a service speed of 22.6 knots (41.9 km/h; 26.0 mph); Queen Mary 2's normal service speed is 26 knots (48 km/h; 30 mph). While the hull of a cruise ship will typically have a block coefficient of 0.73 (1.0 would represent a rectangular block) Queen Mary 2 is more fine-lined, with a block coefficient of 0.61.

Cunard completed a design for a new class of 84,000 GT, 2,000 passenger liners on 8 June 1998, but revised them upon comparing those specifications with Carnival Cruise Line's 100,000 GT Destiny-class cruise ships and Royal Caribbean International's 137,276 GT Voyager class.