A sleeper agent is a spy who is placed in a target country or organization, not to undertake an immediate mission, but rather to act as a potential asset if activated. Sleeper agents are popular plot devices in fiction, in particular espionage fiction and science fiction.

In espionage, a sleeper agent is one who has infiltrated into the target country and 'gone to sleep', sometimes for many years. That is, he or she does nothing to communicate with his or her sponsor or any existing agents, nor to obtain information beyond that in public sources. They can also be referred to as 'deep cover' agents. They acquire jobs and identities—ideally ones which will prove useful in the future—and attempt to blend into everyday life as normal citizens. Counter-espionage agencies in the target country cannot, in practice, closely watch all of those who might possibly have been recruited some time before.

In a sense, the best sleeper agents are those who do not need to be paid by the sponsor as they are able to earn enough money to finance themselves. This avoids any possibly traceable payments from abroad. In such cases, it is possible that the sleeper agent might be successful enough to become what is sometimes termed an 'agent of influence'.