Erectile dysfunction (ED or "male impotence") is a sexual dysfunction characterized by the inability to develop or maintain an erection of the penis sufficient for satisfactory sexual performance.

An erection occurs as a hydraulic effect due to blood entering and being retained in sponge-like bodies within the penis. The process is most often initiated as a result of sexual arousal, when signals are transmitted from the brain to nerves in the pelvis. Erectile dysfunction is indicated when an erection is consistently difficult or impossible to produce, despite arousal. There are various and often multiple underlying causes, some of which are treatable medical conditions. The most important organic causes are cardiovascular disease and diabetes, neurological problems (for example, trauma from prostatectomy surgery), hormonal insufficiencies (hypogonadism) and drug side effects. It is important to realize that erectile dysfunction can signal underlying risk for cardiovascular disease.

There is often a contributing and complicating and sometimes a primary psychological or relational problem. Psychological impotence is where erection or penetration fails due to thoughts or feelings (psychological reasons) rather than physical impossibility; this can often be helped. Notably in psychological impotence, there is a strong response to placebo treatment. Erectile dysfunction, tied closely as it is to cultural notions of potency, success and masculinity, can have severe psychological consequences. There is a strong culture of silence and inability to discuss the matter. In reality, it has been estimated that around 1 in 10 men will experience recurring impotence problems at some point in their lives.

Besides treating the underlying causes and psychological consequences, the first line treatment of erectile dysfunction consists of a trial of PDE5 inhibitor drugs (the first of which was sildenafil or Viagra). In some cases, treatment can involve prostaglandin tablets in the urethra, intracavernous injections with a fine needle into the penis that cause swelling, a penile prosthesis, a penis pump or vascular reconstructive surgery.

The Latin term impotentia coeundi describes simple inability to insert the penis into the vagina. It is now mostly replaced by more precise terms. The study of erectile dysfunction within medicine is covered by andrology, a sub-field within urology.