Leviticus 18 is a chapter of the Biblical book of Leviticus. It narrates part of the instructions given to Moses by God on Mount Sinai. The chapter deals with a number of sexual activities considered 'unclean' or 'abominable'. Although the chapter is principally concerned with incest, it also contains laws related to bestiality and "lying with a man as with a woman." This single reference in verse 22 has, in recent years, made the interpretation of this verse a focus of debate among Christians and Jews regarding homosexual activity (see Homosexuality and Christianity and Jewish views of homosexuality).

Leviticus 18 is generally regarded as part of the holiness code of Leviticus 11–26, and its sexual prohibitions are largely paralleled by Leviticus 20 (except that chapter has more emphasis on punishment).

The Bible lists several types of relationship which it regards as incestuous unions; one list appears in the Deuteronomic Code, and two lists occur in the Holiness Code of Leviticus. These lists only mention relationships with female relatives; excluding lesbianism, this implies that the list is addressed to men. Since the lists would then describe women with whom it is forbidden for a man to have a relationship, they also indirectly imply a list of men with whom it is forbidden for a woman to have a relationship. These lists then compare as follows (blue = forbidden for men only, pink = forbidden for women only, purple = forbidden for both men and women):