A ship classification society or ship classification organisation is a non-governmental organization that establishes and maintains technical standards for the construction and operation of ships and offshore structures. Classification societies certify that the construction of a vessel comply with relevant standards and carry out regular surveys in service to ensure continuing compliance with the standards. Currently, more than 50 organizations describe their activities as including marine classification, twelve of which are members of the International Association of Classification Societies.

A classification certificate issued by a classification society recognised by the proposed ship register is required for a ship's owner to be able to register the ship and to obtain marine insurance on the ship, and may be required to be produced before a ship's entry into some ports or waterways, and may be of interest to charterers and potential buyers. To avoid liability, classification societies explicitly disclaim responsibility for the safety, fitness for purpose, or seaworthiness of the ship, but is a verification only that the vessel is in compliance with the classification standards of the society issuing the classification certificate.

Classification societies also issue International Load Line Certificates in accordance with the legislation of participating states giving effect to the International Convention on Load Lines (CLL 66/88).