IMO number

A seven-digit identifier assigned to a ship's hull for life by the International Maritime Organization, which does not change with name, owner or flag.

reviewed: 2026-08-20

Because the IMO number stays with the hull, it is the only stable identifier a vessel has. Names change, flags change, registered owners change, and the number does not.

That is exactly why it appears in designation entries and why physical alteration of the markings, or transmission of a different number over AIS, is treated as a strong indicator rather than an administrative error. Vessel-level records keyed to IMO numbers are outside the scope of this site.

Where this term is used

What is AIS manipulation and AIS spoofing? — Disabling a ship's position transponder, or transmitting false position data through it, so that the vessel's recorded track does not show where it actually went.
What is flag hopping? — Re-registering a ship repeatedly between flag states, often to registries with limited oversight, so that its regulatory history is broken and its documented identity keeps changing.
What is a ship-to-ship transfer? — Transferring cargo directly between two vessels at sea, rather than through a port, so that the cargo changes hands where there is no port authority, no customs entry and no independent record.

Related terms

Automatic Identification System — A mandatory maritime transponder system that broadcasts a vessel's identity, position, course and speed to nearby vessels and shore stations.
Flag state — The country in which a ship is registered, whose law governs the vessel and which is responsible for inspecting and certifying it.
Flag of convenience — Registration of a ship in a state with which its owners have no genuine connection, chosen for cost, tax or regulatory reasons.

Sources

Guidance to Address Illicit Shipping and Sanctions Evasion Practices. U.S. Department of State, U.S. Department of the Treasury and U.S. Coast Guard, 2020. Global maritime advisory, 14 May 2020.
Reports of the Panel of Experts, 1718 Committee (Democratic People's Republic of Korea). United Nations Security Council, 2024.

All glossary terms