A supernumerary nipple is, quite simply, an extra nipple. In humans, the supernumerary nipple takes the form of a third nipple. Mammals with more than two nipples can also develop supernumerary nipples. As a general rule, supernumerary nipples are entirely harmless, and no medical action needs to be taken to deal with them. Some people have their extra nipples removed for aesthetic reasons, however.

These additional nipples appear along the “milk lines,” areas of the body where the skin is thicker, allowing the development of breast tissue and nipples to occur. Supernumerary nipples can appear anywhere from the head down to the feet, and they may vary considerably in composition. Some, for example, are fully formed nipples with a small amount of breast tissue, capable of potentially producing milk, while others are much more rudimentary. In some cases, the supernumerary nipple may simply be a little tuft of hair.

Often, supernumerary nipples are mistaken for moles, and people are surprised to learn that they actually have extra nipples. In humans, supernumerary nipples are sometimes referred to as thripples, third nipples, accessory nipples, or polymastia. They arise during fetal development, when something along the milk lines goes slightly awry, triggering the development of an additional nipple. Around one in 18 humans has an extra nipple, making the condition not at all unusual.

Although people most commonly think of a third nipple when they hear of extra nipples, it is in fact possible to develop multiple supernumerary nipples. The extra nipple is often visible at birth, and it changes little over the course of a person's life, usually not developing significant breast tissue in puberty. However, fully-formed supernumerary nipples can potentially be at risk of breast cancer, and in people who are diagnosed with breast cancer, the supernumerary nipples may be removed, just to be on the safe side.

Removal of a supernumerary nipple is usually a relatively simple task, since the area is small. Some stitches may be taken or a skin graft may be applied, and the area is typically sore for a week or so while the early stages of healing begin. A small scar may be left behind by the removal, especially if the supernumerary nipple was large and especially well-formed.

Some people actually enjoy their extra nipples, feeling that the supernumerary nipple confers a bit of a special status. Like other unusual markings which appear in the course of fetal development, supernumerary nipples have been viewed with superstition historically. In some cultures, having a third nipple was supposed to be lucky, while in other instances it was viewed as a sign of witchcraft or heresy.