Rational emotive behavior therapy (REBT), previously called rational therapy and rational emotive therapy, is an active-directive, philosophically and empirically based psychotherapy, the aim of which is to resolve emotional and behavioral problems and disturbances and to help people to lead happier and more fulfilling lives.

REBT posits that people have erroneous beliefs about situations they are involved in, and that these beliefs cause disturbance, but can be disputed and changed.

Rational emotive behavior therapy was created and developed by the American psychotherapist and psychologist Albert Ellis, who was inspired by many of the teachings of Asian, Greek, Roman and modern philosophers. REBT is a form of cognitive behavioral therapy (CBT) and was first expounded by Ellis in the mid-1950s; development continued until his death in 2007. Ellis became synonymous with the highly influential therapy. Psychology Today noted, "No individual—not even Freud himself—has had a greater impact on modern psychotherapy."

REBT is both a psychotherapeutic system of theory and practices and a school of thought established by Ellis. He first presented his ideas at a conference of the American Psychological Association in 1956 then published a seminal article in 1957 entitled "Rational psychotherapy and individual psychology", in which he set the foundation for what he was calling rational therapy (RT) and carefully responded to questions from Rudolf Dreikurs and others about the similarities and differences with Alfred Adler's individual psychology. This was around a decade before psychiatrist Aaron Beck first set forth his "cognitive therapy", after Ellis had contacted him in the mid-1960s. Ellis' own approach was renamed Rational Emotive Therapy in 1959, then the current term in 1992.

Precursors of certain fundamental aspects of rational emotive behavior therapy have been identified in ancient philosophical traditions, particularly to Stoics Marcus Aurelius, Epictetus, Zeno of Citium, Chrysippus, Panaetius of Rhodes, Cicero, and Seneca, and early Asian philosophers Confucius and Gautama Buddha. In his first major book on rational therapy, Ellis wrote that the central principle of his approach, that people are rarely emotionally affected by external events but rather by their thinking about such events, "was originally discovered and stated by the ancient Stoic philosophers." Ellis illustrates this with a quote from the Enchiridion of Epictetus: "Men are disturbed not by things, but by the views which they take of them." Ellis noted that Shakespeare expressed a similar thought in Hamlet: "There's nothing good or bad but thinking makes it so." Ellis also acknowledges early 20th century therapists, particularly Paul Charles Dubois, though he only read his work several years after developing his therapy.

The REBT framework posits that humans have both innate rational (meaning self-helping, socially helping, and constructive) and irrational (meaning self-defeating, socially defeating, and unhelpful) tendencies and leanings. REBT claims that people to a large degree consciously and unconsciously construct emotional difficulties such as self-blame, self-pity, clinical anger, hurt, guilt, shame, depression and anxiety, and behavior tendencies like procrastination, compulsiveness, avoidance, addiction and withdrawal by the means of their irrational and self-defeating thinking, emoting and behaving.

REBT is then applied as an educational process in which the therapist often active-directively teaches the client how to identify irrational and self-defeating beliefs and philosophies which in nature are rigid, extreme, unrealistic, illogical and absolutist, and then to forcefully and actively question and dispute them and replace them with more rational and self-helping ones. By using different cognitive, emotive and behavioral methods and activities, the client, together with help from the therapist and in homework exercises, can gain a more rational, self-helping and constructive rational way of thinking, emoting and behaving.

One of the main objectives in REBT is to show the client that whenever unpleasant and unfortunate activating events occur in people's lives, they have a choice between making themselves feel healthily or, self-helpingly, sorry, disappointed, frustrated, and annoyed or making themselves feel unhealthily and self-defeatingly horrified, terrified, panicked, depressed, self-hating and self-pitying. By attaining and ingraining a more rational and self-constructive philosophy of themselves, others and the world, people often are more likely to behave and emote in more life-serving and adaptive ways.