IMO Number
An IMO Number is a unique seven-digit identifier permanently assigned to a merchant ship for its entire life, under the IMO ship identification scheme. Crucially, the number never changes — it stays with the hull regardless of changes of owner, name, flag or type — providing a stable, unambiguous way to identify a specific vessel over time.
Displayed as “IMO 1234567” (with a check digit), it is used throughout shipping for registration, class, port state control, safety and pollution records, casualty investigation, and vessel tracking and databases. Because names and flags change frequently but the IMO number does not, it is the reliable key for identifying and tracking a ship's history — essential for safety, transparency, sanctions compliance and preventing fraud. It complements other identifiers such as the MMSI (used by AIS/radio, which can change) and the ship's call sign. In vessel and fleet systems, the IMO number is the definitive vessel identifier. Companies also have an IMO company number. The IMO number is a foundational reference for identifying ships unambiguously across the industry.
Ships change name, flag and owner constantly, which makes tracking a specific vessel's history hard — the IMO number solves it, staying fixed to the hull for life. That permanence makes it the reliable key for safety records, port state control, sanctions compliance and anti-fraud, and the definitive vessel identifier in any fleet or tracking system.
Does a ship's IMO number change if it is sold or renamed?
No — the IMO number is permanent for the life of the hull, regardless of changes of owner, name, flag or type.
How does the IMO number differ from the MMSI?
The IMO number permanently identifies the ship; the MMSI is a radio/AIS identifier that can change with flag or ownership.