Pederasty or paederasty (/ˈpɛdəræsti/) is a sexual relationship between an adult man and an adolescent boy. It was a socially acknowledged practice in Ancient Greece and Rome and elsewhere in the world, such as Pre-Meiji Japan. Less frequently, it involved an adult woman and an adolescent girl.

In most countries today, the local age of consent determines whether a person is considered legally competent to consent to sexual acts, and whether such contact is child sexual abuse or statutory rape. An adult engaging in sexual activity with a minor is considered abusive by authorities for a variety of reasons, including the age of the minor and the psychological and physical harm they may endure.

Pederasty derives from the combination of Ancient Greek: παίδ-, romanized: paid-, lit. 'boy, child (stem)' with ἐραστής, erastēs, 'lover' (cf. eros). Late Latin pæderasta was borrowed in the 16th century directly from Plato's classical Greek in The Symposium. (Latin transliterates αί as æ.) The word first appeared in the English language during the Renaissance, as pæderastie (e.g. in Samuel Purchas' Pilgrimes), in the sense of sexual relations between men and boys.

The Oxford English Dictionary defines it as "Homosexual relations between a man and a boy; homosexual anal intercourse, usually with a boy or younger man as the passive partner".

Pederasty in ancient Greece was a socially acknowledged romantic relationship between an adult male (the erastes) and a younger male (the eromenos), usually in his teens. This age difference between a socially powerful and socially less-powerful partner was characteristic of the Archaic and Classical periods, in both heterosexual and homosexual relationships. The influence of pederasty on Greek culture of these periods was so pervasive that it has been called "the principal cultural model for free relationships between citizens." The practice was viewed with concerns and disapproval by certain social groups.

In the writings of Xenophon, Socrates says, "A man who sells his favours for a price to anyone who wants them is called a catamite; but if anyone forms a love-attachment with someone whom he knows to be truly good, we regard him as perfectly respectable." Each author may have used Socrates as a spokesman for their own viewpoints. Both Xenophon's Socrates and Plato's Socrates were against homosexual copulation in the context of pederasty and instead advocated for platonic relationships. The Socratic writings of the two authors were one of the main texts that led to Kenneth Dover's and Michel Foucault's understanding of pederasty as a matter of debate in Ancient Greece.

Some scholars locate its origin in initiation ritual, particularly rites of passage on Crete, where it was associated with entrance into military life and the religion of Zeus. It has no formal existence in the Homeric epics, and seems to have developed in the late 7th century BC as an aspect of Greek homosocial culture, which was characterized also by athletic and artistic nudity, delayed marriage for aristocrats, symposia, and the social seclusion of women. Pederasty was both idealized and criticized in ancient literature and philosophy. The argument has recently been made that idealization was universal in the Archaic period; criticism began in Athens as part of the general Classical Athenian reassessment of Archaic culture.

Scholars have debated the role or extent of pederasty, which is likely to have varied according to local custom and individual inclination. Athenian law, for instance, recognized both consent and age as factors in regulating sexual behavior.