Polygamy (from Late Greek πολυγαμία (polugamía) "state of marriage to many spouses") is the practice of marrying multiple spouses. When a man is married to more than one wife at the same time, sociologists call this polygyny. When a woman is married to more than one husband at a time, it is called polyandry.

In contrast to polygamy, monogamy is marriage consisting of only two parties. Like "monogamy", the term "polygamy" is often used in a de facto sense, applied regardless of whether a state recognizes the relationship. In sociobiology and zoology, researchers use polygamy in a broad sense to mean any form of multiple mating.

Worldwide, different societies variously encourage, accept or outlaw polygamy. In societies which allow or tolerate polygamy, in the vast majority of cases the form accepted is polygyny. According to the Ethnographic Atlas Codebook (1998), of 1,231 societies noted, 588 had frequent polygyny, 453 had occasional polygyny, 186 were monogamous and 4 had polyandry – although more recent research suggests that polyandry may occur more commonly than previously thought. In cultures which practice polygamy, its prevalence among that population often correlates with social class and socioeconomic status.

From a legal point of view, in many countries, although the law only recognises monogamous marriages (a person can only have one spouse, and bigamy is illegal), adultery is not illegal, leading to a situation of de facto polygamy being allowed, although without legal recognition for non-official "spouses".

Scientific studies classify the human mating system as primarily monogamous, with the cultural practice of polygamy in the minority, based both on surveys of world populations, and on characteristics of human reproductive physiology.

Polygamy (taking the form of polygyny) is most common in a region known as the "polygamy belt" in West Africa and Central Africa, with the countries estimated to have the highest polygamy prevalence in the world being Burkina Faso, Mali, Gambia, Niger and Nigeria.