Priapism is a potentially harmful medical condition in which the erect penis does not return to its flaccid state (despite the absence of both physical and psychological stimulation) within about four hours. It is often painful. Priapism is considered a medical emergency, which should receive proper treatment by a qualified medical practitioner.

The name comes from the Greek god Priapus, referring to the myth that he was ironically punished by the other gods for attempting to rape a goddess, by being given a huge (but useless) set of wooden genitals.

The causative mechanisms are poorly understood but involve complex neurological and vascular factors. Priapism may be associated with haematological disorders, especially sickle-cell disease, and other conditions such as leukemia, thalassemia, and Fabry's disease, and neurologic disorders such as spinal cord lesions and spinal cord trauma (priapism has been reported in hanging victims; see death erection).

Priapism can be caused by medications. The most common medications that cause priapism are intra-cavernous injections for treatment of erectile dysfunction (papaverine, alprostadil). Other groups reported are antihypertensives, antipsychotics (e.g chlorpromazine, clozapine), antidepressants (most notably trazodone), anticoagulants, cantharides (Spanish Fly) and recreational drugs (alcohol and cocaine). Phosphodiesterase type-5 (PDE5) inhibitors such as sildenafil (popularly Viagra), tadalafil and vardenafil have very rarely been implicated, and probably do not cause priapism. PDE-5 inhibitors have even been evaluated as preventive treatment for recurrent priapism.

Potential complications include ischemia, clotting of the blood retained in the penis (thrombosis), and damage to the blood vessels of the penis which may result in an impaired erectile function or impotence. In serious cases, the ischemia may result in gangrene, which could necessitate penis removal.