Betrayal is the breaking or violation of a presumptive contract, trust, or confidence that produces moral and psychological conflict within a relationship amongst individuals, between organizations or between individuals and organizations. Often betrayal is the act of supporting a rival group, or it is a complete break from previously decided upon or presumed norms by one party from the others. Someone who betrays others is commonly known as a traitor or betrayer.

Betrayal is a commonly used story element in fiction, sometimes used as a plot twist.

Philosophers Judith Shklar and Peter Johnson, authors of The Ambiguities of Betrayal and Frames of Deceit, respectively, contend that while no clear definition of betrayal is available, betrayal is more effectively understood through literature.

Rodger L. Jackson explains why a clear definition is needed:

Betrayal is both a "people" problem and a philosopher's problem. Philosophers should be able to clarify the concept of betrayal, compare and contrast it with other moral concepts, and critically assess betrayal situations. At the practical level people should be able to make honest sense of betrayal and also to temper its consequences: to handle it, not be assaulted by it. What we need is a conceptually clear account of betrayal that differentiates between genuine and merely perceived betrayal, and which also provides systematic guidance for the assessment of alleged betrayal in real life.

An act of betrayal creates a constellation of negative behaviours, thoughts, and feelings in both its victims and its perpetrators. Accepting the betrayal and going no contact is seen by some as the best route forward. The alternative is to stay in connection and realize the trespass can happen again, and may choose to avoid doing certain things to decrease severity.

Betrayal trauma has symptoms similar to posttraumatic stress disorder, although the element of amnesia and dissociation is likely to be greater.

The key difference between traditional posttraumatic stress disorder (PTSD) and betrayal trauma is that the former is historically seen as being caused primarily by fear, whereas betrayal trauma is a response to extreme anger. Another key difference is that betrayal trauma involves an individual experiencing a violation of trust between a trusted individual or institution, whereas posttraumatic stress disorder does not involve a violation from a trusted source.