Ganser syndrome is a rare dissociative disorder characterized by nonsensical or wrong answers to questions and other dissociative symptoms such as fugue, amnesia or conversion disorder, often with visual pseudohallucinations and a decreased state of consciousness. The syndrome has also been called nonsense syndrome, balderdash syndrome, syndrome of approximate answers, hysterical pseudodementia or prison psychosis.

The term prison psychosis is sometimes used because the syndrome occurs most frequently in prison inmates, where it may be seen as an attempt to gain leniency from prison or court officials. Psychological symptoms generally resemble the patient's sense of mental illness rather than any recognized category. The syndrome may occur in persons with other mental disorders such as schizophrenia, depressive disorders, toxic states, paresis, alcohol use disorders and factitious disorders. Ganser syndrome can sometimes be diagnosed as merely malingering, but it is more often defined as dissociative disorder.

The identification of Ganser syndrome is attributed to German psychiatrist Sigbert Ganser (1853–1931). In 1898, he described the disorder in prisoners awaiting trial in a penal institution in Halle, Germany. He named impaired consciousness and distorted communication, namely in the form of approximate answers (also referred to as Vorbeireden in the literature), as the defining symptoms of the syndrome. Vorbeireden involves the inability to answer questions precisely, although the content of the questions is understood.

Ganser syndrome is described as a dissociative disorder not otherwise specified (NOS) in the DSM-IV, and is not currently listed in the DSM-5. It is a rare and an often overlooked clinical phenomenon. In most cases, it is preceded by extreme stress and followed by amnesia for the period of psychosis. In addition to approximate answers, other symptoms include a clouding of consciousness, somatic conversion disorder symptoms, confusion, stress, loss of personal identity, echolalia, and echopraxia.