Polygyny (from neo-Greek: πολύ poly - "many", and γυνή gyny - "woman or wife") is a form of marriage in which a man has two or more wives at the same time. In countries where the practice is illegal, the man is referred to as a bigamist or a polygamist. It is distinguished from relationships where a man has a sexual partner outside marriage, such as a concubine, casual sexual partner, paramour, cohabits with a married woman or other culturally but not legally recognized secondary partner. Polygyny is the most common form of polygamy; the much rarer practice of polyandry is the form of marriage in which one woman has two or more husbands at the same time.

Polygyny is generally studied by anthropologists rather than by sociologists. Thus, nearly all studies of African polygyny have treated it as a traditional form of marriage inevitably destined to disappear under the pressures of urbanization, wage employment, Christianity and general "modernization" it would appear that no African woman has written in praise of polygyny, although some have accepted it as a lesser evil given the probable alternative of statusless concubinage. The issue of women's perceptions of polygyny has been gravely neglected.