Endogamy is a cultural practice of marrying within a specific social group, religious denomination, caste, or ethnic group, rejecting those from others as unsuitable for marriage or other close interpersonal relationships. It is the opposite of exogamy, which describes the social norm of marriage outside of the group. Endogamy has been common among extant and historical aristocracies, religious groups, ethnic groups, and social classes. The penalties for transgressing endogamous restrictions have varied greatly among cultures and have ranged from death to mild disapproval.

Endogamy is characteristic of aristocracies and religious and ethnic minorities in industrialized societies but also of the caste system in India and of class. Expectations of caste endogamy persist in parts of India and the Indian diaspora, although many claim that this is a form of caste discrimination, a practice made illegal in the mid-20th century. Several religious and ethnic religious groups are traditionally more endogamous, although sometimes with the added dimension of requiring marital religious conversion.

Endogamy can be the result of a conscious decision or cultural pressure to marry within the selected group but also occurs as a result of geographical isolation (for example, in island communities). A long-term pattern of endogamy in a region may increase the risk of repeated cousin marriage during a long period of time, referred to as inbreeding. It may cause additional noise in the DNA autosomal data, giving the impression that DNA matches with roots in that region are more closely related than they are. Examples of ethnic and religious groups that have typically practiced endogamy include Alawites, the Amish of North America, various Arab tribes, Assyrians, Armenians, and Jews.