A great seal is a seal used by a head of state, or someone authorised to do so on their behalf, to confirm formal documents, such as laws, treaties, appointments and letters of dispatch. It was and is used as a guarantee of the authenticity of the most important and solemn records and documents.

In the Middle Ages, the great seal played a far greater political role and there was much real power associated with having control over the seal. In addition to the great seal, a prince usually also had a privy seal, used for correspondence of a more private nature.

The seal is usually formally entrusted upon an office-holder to act as keeper of the seal. This keeper may be a separate office but was also usually combined with that of the chancellor. Nowadays, the great seal is usually entrusted upon a minister, particularly a minister of justice.