A secret society is a social organization that requires its members to conceal certain activities - such as rites of initiation or club ceremonies - from outsiders. Members may be required to conceal or deny their membership, and are often sworn to hold the society's secrets by an oath. The term "secret society" is often used to describe fraternal organizations (e.g. Freemasonry) that may have secret ceremonies, but is also commonly applied to organizations ranging from the common and innocuous (collegiate fraternities) to mythical organizations described in conspiracy theories as immensely powerful, with self-serving financial or political agendas with global reach.

Historically, secret societies are often the subject of suspicion and speculation from non-members, and as such have aroused nervousness from outsiders since at least the time of the ancient Greeks. For this reason, secret societies are illegal in several countries. In the European Union, Poland has made the ban a part of its constitution.

Some secret organizations exploit secrecy as a means to further political or criminal agendas, including such historical examples as the Know Nothing party in the United States, and the Mafia, respectively.