Depersonalization-derealization disorder (DPDR, DDD) is a mental disorder in which the person has persistent or recurrent feelings of depersonalization and/or derealization. Depersonalization is described as feeling disconnected or detached from one's self. Individuals may report feeling as if they are an outside observer of their own thoughts or body, and often report feeling a loss of control over their thoughts or actions. Derealization is described as detachment from one's surroundings. Individuals experiencing derealization may report perceiving the world around them as foggy, dreamlike, surreal, and/or visually distorted.

Depersonalization-derealization disorder is thought to be caused largely by interpersonal trauma such as early childhood abuse. Adverse childhood experiences, specifically emotional abuse and neglect, have been linked to the development of depersonalization symptoms. Feelings of depersonalization and derealization are common from significant stress or panic attacks. Individuals may remain in a depersonalized state for the duration of a typical panic attack. However, in some cases, the dissociated state may last for hours, days, weeks, or even months at a time. In rare cases, symptoms of a single episode can last for years.

Diagnostic criteria for depersonalization-derealization disorder includes persistent or recurrent feelings of detachment from one's mental or bodily processes or from one's surroundings. A diagnosis is made when the dissociation is persistent, interferes with the social or occupational functions of daily life, and/or causes marked distress in the patient.

While depersonalization-derealization disorder was once considered rare, lifetime experiences with it occur in about 1–2% of the general population. The chronic form of the disorder has a reported prevalence of 0.8 to 1.9%. While brief episodes of depersonalization or derealization can be common in the general population, the disorder is only diagnosed when these symptoms cause substantial distress or impair social, occupational, or other important areas of functioning.

The core symptoms of depersonalization-derealization disorder are the subjective experience of "unreality in one's self", or detachment from one's surroundings. People who are diagnosed with depersonalization also often experience an urge to question and think critically about the nature of reality and existence.

Individuals with depersonalization describe feeling disconnected from their physicality; feeling as if they are not completely occupying their own body; feeling as if their speech or physical movements are out of their control; feeling detached from their own thoughts or emotions; and experiencing themselves and their lives from a distance. While depersonalization involves detachment from one's self, individuals with derealization feel detached from their surroundings, as if the world around them is foggy, dreamlike, or visually distorted. Individuals with the disorder commonly describe a feeling as though time is passing them by and they are not in the notion of the present. In some cases, individuals may be unable to accept their reflection as their own, or they may have out-of-body experiences. Additionally some individuals experience difficulty concentrating and problems with memory retrieval. These individuals sometimes lack the "feeling" of a memory where they are able to recall a memory but feel as if they did not personally experience it. These experiences which strike at the core of a person's identity and consciousness may cause a person to feel uneasy or anxious. The inner turmoil created by the disorder can also result in depression.

First experiences with depersonalization may be frightening, with patients fearing loss of control, dissociation from the rest of society and functional impairment. The majority of people with depersonalization-derealization disorder misinterpret the symptoms, thinking that they are signs of serious psychosis or brain dysfunction. This commonly leads to an increase of anxiety and obsession, which contributes to the worsening of symptoms.

Factors that tend to diminish symptoms are comforting personal interactions, intense physical or emotional stimulation, and relaxation. Distracting oneself (by engaging in conversation or watching a movie, for example) may also provide temporary relief. Some other factors that are identified as relieving symptom severity are diet or exercise, while alcohol and fatigue are listed by some as worsening their symptoms.