Endogamy is the cultural practice of mating - usually in the form of marriage - within a specific social group, religious denomination, caste, or ethnic group, rejecting those from others as unsuitable for marriage or other close interpersonal relationships. Whereas endogamy refers to marriage within the group, its opposite, exogamy, describes the social norm of marriage outside of the group.

Endogamy is common in many cultures and ethnic groups. Several religious and ethnic religious groups are traditionally more endogamous, although sometimes with the added dimension of requiring marital religious conversion. This permits an exogamous marriage, as the convert, by accepting the partner's religion, becomes accepted within the endogamous rules. Endogamy, as distinct from consanguinity, may result in transmission of genetic disorders, the so-called founder effect, within the relatively closed community.