A functional symptom is a medical symptom with no known physical cause. In other words, there is no structural or pathologically defined disease to explain the symptom. The use of the term 'functional symptom' does not assume psychogenesis, only that the body is not functioning as expected. Functional symptoms are increasingly viewed within a framework in which 'biological, psychological, interpersonal and healthcare factors' should all be considered to be relevant for determining the aetiology and treatment plans.

Historically, there has often been fierce debate about whether certain problems are predominantly related to an abnormality of structure (disease) or are psychosomatic in nature, and what are at one stage posited to be functional symptoms are sometimes later reclassified as organic, as investigative techniques improve. It is well established that psychosomatic symptoms are a real phenomenon, so this potential explanation is often plausible, however the commonality of a range of psychological symptoms and functional weakness does not imply that one causes the other. For example, symptoms associated with migraine, epilepsy, schizophrenia, multiple sclerosis, stomach ulcers, chronic fatigue syndrome, Lyme disease and many other conditions have all tended historically at first to be explained largely as physical manifestations of the patient's psychological state of mind; until such time as new physiological knowledge is eventually gained. Another specific example is functional constipation, which may have psychological or psychiatric causes. However, one type of apparently functional constipation, anismus, may have a neurological (physical) basis.

Whilst misdiagnosis of functional symptoms does occur, in neurology, for example, this appears to occur no more frequently than of other neurological or psychiatric syndromes. However, in order to be quantified, misdiagnosis has to be recognized as such, which can be problematic in such a challenging field as medicine.

A common trend is to see functional symptoms and syndromes such as fibromyalgia, irritable bowel syndrome and functional neurological symptoms such as functional weakness as symptoms in which both biological and psychological factors are relevant, without one necessarily being dominant.