A triple agent is a type of spy who uses his or her intelligence connections to support the aims of a particular intelligence agency while embedded in another agency. Being a triple agent is extremely dangerous, as the uncovering of a triple agent could potentially be life threatening or at least disastrous. Before someone works as a triple agent, his or her background is usually carefully investigated, and a complex system of communications and handlers is established.

Before explaining precisely how a triple agent works, it can help to understand what a double agent is. A double agent is someone who pretends to spy on agency X on behalf of agency Y while being actually loyal to agency X. The double agent is used to feed misinformation to agency Y, ensuring that the agency has incomplete or inaccurate information on the opposing agency. Many double agents pretend to be defectors, approaching agency Y with claims of useful information on the orders of their handlers at agency X.

A triple agent is someone who pretends to be a double agent, telling agency X that his or her loyalties lie with that agency while actually being loyal to agency Y. Through the communication channels established and monitored by agency X, the double agent will feed misinformation to agency Y to make his or her position as a double agent appear legitimate, while providing actual information through a totally separate and isolated channel. The triple agent will also bring back bits of information about agency Y to keep agency X satisfied that the triple agent is still loyal.

This form of espionage can get extremely complex, as it requires either successfully embedding an agent in an enemy agency, or managing to recruit someone out of an opposing agency. Very careful steps must be taken to protect the safety and identity of the triple agent, and, on occasion, the agent's information may not be usable because it could lead to exposure. However, it can still be very valuable, providing an extensive picture of the opposing agency's practices and plans.

While it is theoretically possible to find what are known as “multiply turned agents,” meaning agents who have even more tangled networks of loyalties, these agents are fairly rare. Many counterintelligence agencies are understandably concerned about the loyalties of triple agents, as the shifts between loyalties can suggest that the agent may in fact be multiply turned. For this reason, triple agents are often put under pressure by the agencies they claim to be working for, in an effort to ensure that they are, in fact, loyal.