The IMO number of the International Maritime Organization is a generic term covering two distinct meanings. The IMO ship identification number is a unique ship identifier; the IMO company and registered owner identification number is used to identify uniquely each company and/or registered owner managing ships of at least 100 gross tons (gt). The schemes are managed in parallel, but IMO company/owner numbers may also be obtained by managers of vessels not having IMO ship numbers. IMO numbers were introduced to improve maritime safety and reduce fraud and pollution, under the International Convention for the Safety of Life at Sea (SOLAS).
The IMO ship number scheme has been mandatory, for SOLAS signatories, for passenger and cargo ships above a certain size since 1996, and voluntarily applicable to various other vessels since 2013/2017. The number identifies a ship and does not change when the ship's owner, country of registry (flag state) or name changes, unlike the official numbers used in some countries, e.g. the UK. The ship's certificates must also bear the IMO ship number. Since 1 July 2004, passenger ships are also required to carry the marking on a horizontal surface visible from the air.