A supernumerary nipple is an additional instance of nipple occurring in mammals, including humans and monkeys. They are often mistaken for moles. Studies variously report the prevalence of supernumerary nipples as approximately 1 in 18 and 1 in 40.

The nipples appear along the two vertical "milk lines", which start in the armpit on each side, run down through the typical nipples and end at the groin. They are classified into eight levels of completeness, from a simple patch of hair to a milk-bearing breast in miniature.

Polythelia refers to the presence of an additional nipple alone, while polymastia denotes the much rarer presence of additional mammary glands.

Although usually presenting on the milk line, pseudomamma can appear as far away as the foot.

It may remain undetected. Occasionally, the supernumerary nipple is noticed when hormonal changes during adolescence, menstruation, or pregnancy cause increased pigmentation, fluctuating swelling, tenderness, or even lactation.

It is said to be found in association with many syndromes and other conditions:

A possible connection with mitral valve prolapse has been proposed.

Most often no treatment is required; however, a protruding embarrassing supernumerary nipple can be removed surgically, if desired. Removal using liquid nitrogen cryotherapy has also been described.