In psychoanalytic theory, a defence mechanism (American English: defense mechanism), is an unconscious psychological operation that functions to protect a person from anxiety-producing thoughts and feelings related to internal conflicts and outer stressors.

Defence mechanisms (German: Abwehrmechanismen) are unconscious psychological processes employed to defend against feelings of anxiety and unacceptable impulses at the level of consciousness. These processes include: repression, the exclusion of unacceptable desires and ideas from consciousness, though in certain circumstances they may resurface in a disguised or distorted form; identification, a form of attachment to an object which results in incorporating some of its aspects into oneself, employed by the ego and superego to attract libido away from objects and toward themselves, thereby fortifying the personality; and rationalization, the justification of one's behaviour and motivations by substituting "good" acceptable reasons for the actual motivations. In psychoanalytic theory, repression is considered the basis for other defence mechanisms.

According to this theory, healthy people normally use different defence mechanisms throughout life. A defence mechanism becomes pathological only when its persistent use leads to maladaptive behaviour such that the physical or mental health of the individual is adversely affected. Among the purposes of ego defence mechanisms is to protect the mind/self/ego from anxiety or social sanctions or to provide a refuge from a situation with which one cannot currently cope.