An ocean liner is a type of passenger ship primarily used for transportation across seas or oceans. Ocean liners may also carry cargo or mail, and may sometimes be used for other purposes (such as for pleasure cruises or as hospital ships). Only one ocean liner remains in service today.

As of 2026, there is only one true ocean liner remaining in active service: the RMS Queen Mary 2 (QM2), operated by Cunard Line. Unlike modern cruise ships, which are built primarily for leisure and tend to be boxy and high-riding, the QM2 is a "purpose-built" ocean liner designed for speed and stability during transatlantic crossings. It features a deeper draft, a more pointed bow, and a hull constructed with 40% more steel than a standard cruise ship to withstand the rough waters of the North Atlantic. While other historic liners like the original Queen Mary in Long Beach and the SS United States in Philadelphia still exist, they are permanently docked as hotels or awaiting restoration and are no longer seaworthy. The QM2 continues to carry the "RMS" (Royal Mail Ship) prefix, maintaining a century-old tradition of regular service between Southampton and New York. It can travel at speeds up to 30 knots, significantly faster than the 20-22 knots typical of modern vacation-oriented cruise vessels.