A mammary gland is an organ in female mammals that produces milk to feed young offspring. Mammals get their name from the word "mammary". In humans, the mammary glands are situated on the breasts. In ruminants such as cows, goats, and deer, the mammary glands are contained in their udders. The mammary glands of other mammals that have more than two breasts, such as dogs and cats, are sometimes called dugs.

A mammary gland is a specific type of apocrine gland specialized for manufacture of colostrum at the time of parturition. Mammary glands can be identified as apocrine because they exhibit striking "decapitation" secretion. Whether mammary glands are modified sweat glands or sebaceous glands still remains controversial.

The basic components of a mature mammary gland are the alveoli (hollow cavities, a few millimetres large) lined with milk-secreting cuboidal cells and surrounded by myoepithelial cells. These alveoli join up to form groups known as lobules, and each lobule has a lactiferous duct that drains into openings in the nipple. The myoepithelial cells can contract under the stimulation of oxytocin thereby excreting milk secreted from alveolar units into the lobule lumen toward the nipple, where it collects in sinuses of the ducts. As the infant begins to suck, the hormonally (oxytocin) mediated "let down reflex" ensues and the mother's milk is secreted – not sucked from the gland – into the baby's mouth.