Polyandry (; from Ancient Greek πολύ (polú) 'many', and ἀνήρ (anḗr) 'man') is a form of polygamy in which a woman takes two or more husbands at the same time. Polyandry is contrasted with polygyny, involving one male and two or more females. If a marriage involves a plural number of "husbands and wives" participants of each gender, then it can be called polygamy, group or conjoint marriage. In its broadest use, polyandry refers to sexual relations with multiple males within or without marriage.

Of the 1,231 societies listed in the 1980 Ethnographic Atlas, 186 were found to be monogamous, 453 had occasional polygyny, 588 had more frequent polygyny, and 4 had polyandry. Polyandry is less rare than this figure suggests, as it considered only those examples found in the Himalayan mountain region (8 societies). More recent studies have found more than 4 other societies practicing polyandry.

Fraternal polyandry is practiced among Tibetans in Nepal and parts of China, in which two or more brothers are married to the same wife, with the wife having equal "sexual access" to them. It is associated with partible paternity, the cultural belief that a child can have more than one father. Several ethnic groups practicing polyandry in India identify their customs with their descent from Draupadi, a central character of the Mahabharta who was married to five brothers, although local practices may not be fraternal themselves.

Polyandry is believed to be more likely in societies with scarce environmental resources. It is believed to limit human population growth and enhance child survival. It is a rare form of marriage that exists not only among peasant families but also among the elite families. For example, polyandry in the Himalayan mountains is related to the scarcity of land. The marriage of all brothers in a family to the same wife allows family land to remain intact and undivided. If every brother married separately and had children, family land would be split into unsustainable small plots. In contrast, very poor persons not owning land were less likely to practice polyandry in Buddhist Ladakh and Zanskar. In Europe, the splitting up of land was prevented through the social practice of impartible inheritance. With most siblings disinherited, many of them became celibate monks and priests.

Polyandrous mating systems are also a common phenomenon in the animal kingdom.