Delusional parasitosis (DP) is a mental disorder in which individuals have a persistent belief that they are infested with living or nonliving pathogens such as parasites, insects, or bugs, when no such infestation is present. They usually report tactile hallucinations known as formication, a sensation resembling insects crawling on or under the skin. Morgellons is considered to be a subtype of this condition, in which individuals have sores that they believe contain harmful fibers.

Delusional parasitosis is classified as a delusional disorder in the Diagnostic and Statistical Manual of Mental Disorders (DSM5). The cause is unknown, but is thought to be related to excess dopamine in the brain. Delusional parasitosis is diagnosed when the delusion is the only symptom of psychosis and the delusion—that cannot be better explained by another condition—has lasted a month or longer. Few individuals with the condition willingly accept treatment, because they do not recognize the illness as a delusion. Antipsychotic medications offer a potential cure, while cognitive behavioral therapy and antidepressants can be used to help alleviate symptoms.

The condition is rare, and is observed twice as often in women as men. The average age of people with the disorder is 57. An alternative name, Ekbom's syndrome, refers to the neurologist Karl-Axel Ekbom, who published seminal accounts of the disease in 1937 and 1938.