A unique seven-digit identifier assigned to a ship for life, used to track its history regardless of flag, owner, or name changes.
In practice
For port state control officers, the IMO number is the primary means of identifying a vessel and linking it to its inspection and detention history in the database. When a ship changes name or flag — sometimes in an attempt to escape a poor compliance record — the IMO number persists, enabling inspectors to access the full history. This continuity of identity is the core purpose of the scheme.
Regulatory detail & full definition
The IMO number is a unique seven-digit identifier assigned to a ship by IHS Markit under a scheme adopted by IMO Resolution A.1078(28). The number is issued to the ship at the time of keel-laying or acquisition of flag state nationality and remains with the vessel for its entire service life, regardless of name changes, flag changes, or ownership changes. It is also assigned to shipowners and managers to enable tracking of corporate entities through name and ownership changes.
For port state control officers, the IMO number is the primary means of identifying a vessel and linking it to its inspection and detention history in the database. When a ship changes name or flag — sometimes in an attempt to escape a poor compliance record — the IMO number persists, enabling inspectors to access the full history. This continuity of identity is the core purpose of the scheme.
IMO numbers must be permanently marked on the ship's hull and keel plate, and displayed on the ship's structural elements in a contrasting colour. The number must appear on the ship's certificates, the Continuous Synopsis Record, and all official communications with IMO. For seafarers, ensuring that the IMO number on all documents matches the hull markings is a basic compliance check during pre-departure verification.