In psychiatry, depersonalization (or derealization) is the experience of feelings of loss of a sense of reality. A sufferer feels that they have changed and the world has become less real - it is vague, dreamlike, or lacking in significance. A sufferer is divorced from both the world and from their own identity and physicality.

The condition is usually found in conjunction with other mental disorders, especially depression or certain neuroses. If reported together with more serious delusions, depersonalization is a sign of schizophrenia - an indication of the disintergration of personality. A sufferer from depersonalization can be especially susceptible to suicide, undertaking the suicidal process calmly and easily without real awareness.

It is not always a serious condition. When otherwise healthy people experience extended periods of extreme danger, after the danger recedes people can experience depersonalization as a reaction. Soldiers in wartime have reported this phenomena.

Existentialists use the term in a different context. The treatment of individuals by other people as if they were objects, or without regard to their feelings, has been termed depersonalization. Determinism has been accused of this.

R. D. Laing used depersonalization to mean a fear of the lost of autonomy in interpersonal relationships by the ontologically insecure.