Dissociative identity disorder (DID), also known as multiple personality disorder, is a mental disorder characterized by at least two distinct and relatively enduring personality states. There is often trouble remembering certain events, beyond what would be explained by ordinary forgetfulness. These states alternately show in a person's behavior. Presentations, however, are variable. Associated conditions often include borderline personality disorder, post traumatic stress disorder, depression, substance misuse disorder, self-harm, or anxiety.

The cause is believed to be childhood trauma.