Koro is a culture bound delusional disorder in which individuals have an overpowering belief that their sex organs are retracting and will disappear, despite the lack of any true longstanding changes to the genitals. Koro is also known as shrinking penis, and it is listed in the Diagnostic and Statistical Manual of Mental Disorders.

The syndrome occurs worldwide, and mass hysteria of genital-shrinkage anxiety has a history in Africa, Asia and Europe. In the United States and Europe, the syndrome is commonly known as genital retraction syndrome.

The condition can be diagnosed through psychological assessment along with physical examination to rule out genuine disorders of the genitalia that could be causing true retraction.

The word was borrowed from Malay and means the head of a turtle (or tortoise), referring to how it looks when they retract their heads into their shells.