Ship commissioning is the act or ceremony of placing a ship in active service and may be regarded as a particular application of the general concepts and practices of project commissioning. The term is most commonly applied to placing a warship in active duty with its country's military forces. The ceremonies involved are often rooted in centuries-old naval tradition.

Ship naming and launching endow a ship hull with her identity, but many milestones remain before she is completed and considered ready to be designated a commissioned ship. The engineering plant, weapon and electronic systems, galley, and other equipment required to transform the new hull into an operating and habitable warship are installed and tested. The prospective commanding officer, ship's officers, the petty officers, and seamen who will form the crew report for training and familiarization with their new ship.

Before commissioning, the new ship undergoes sea trials to identify any deficiencies needing correction. The preparation and readiness time between christening-launching and commissioning may be as much as three years for a nuclear-powered aircraft carrier to as brief as twenty days for a World War II landing ship. USS Monitor, of American Civil War fame, was commissioned less than three weeks after launch.