Sea cadets are members of a sea cadet corps, a formal uniformed youth organisation for young people with an interest in waterborne activities and or the national navy. The organisation may be sponsored in whole or in part by the navy or a naval supporter's organisation. In the United Kingdom, sea cadets are governed by the parent charity MSSC (Marine Society & Sea Cadets) and receives just over half of its funding from the Ministry of Defence. The Royal Navy is its principal supporter, but it is not a pre-service organisation and works in partnerships with the broader maritime community as well. The various organisations are listed in alphabetical order of their nation.

Sea cadet organisations exist in most of the maritime nations of the world. As described by the International Sea Cadet Association:

A Sea cadet corps or corresponding organisation is a voluntary, non-political and non-militant youth organisation, with membership unrestricted by race, sex or philosophical or religious convictions, which offers practical and theoretical training in nautical and maritime subjects within the context based on naval traditions.