Adultery (from Latin adulterium) is extramarital sex that is considered objectionable on social, religious, moral, or legal grounds. Although the sexual activities that constitute adultery vary, as well as the social, religious, and legal consequences, the concept exists in many cultures and is similar in Christianity, Islam, and Judaism. A single act of sexual intercourse is generally sufficient to constitute adultery, and a more long-term sexual relationship is sometimes referred to as an affair.

Historically, many cultures considered adultery a very serious crime, some subject to severe punishment, usually for the woman and sometimes for the man, with penalties including capital punishment, mutilation, or torture. Such punishments have gradually fallen into disfavor, especially in Western countries from the 19th century. In countries where adultery is still a criminal offense, punishments range from fines to caning and even capital punishment. Since the 20th century, criminal laws against adultery have become controversial, with most Western countries decriminalizing adultery.

However, even in jurisdictions that have decriminalized adultery, it may still have legal consequences, particularly in jurisdictions with fault-based divorce laws, where adultery almost always constitutes a ground for divorce and may be a factor in property settlement, the custody of children, the denial of alimony, etc. Adultery is not a ground for divorce in jurisdictions which have adopted a no-fault divorce model.

International organizations have called for the decriminalization of adultery, especially in the light of several high-profile stoning cases that have occurred in some countries. The head of the United Nations expert body charged with identifying ways to eliminate laws that discriminate against women or are discriminatory to them in terms of implementation or impact, Kamala Chandrakirana, has stated that: "Adultery must not be classified as a criminal offence at all". A joint statement by the United Nations Working Group on discrimination against women in law and in practice states that: "Adultery as a criminal offence violates women’s human rights".

In Muslim countries that follow Sharia law for criminal justice, the punishment for adultery may be stoning. There are fifteen countries in which stoning is authorized as lawful punishment, though in recent times it has been legally carried out only in Iran and Somalia. Most countries that criminalize adultery are those where the dominant religion is Islam, and several Sub-Saharan African Christian-majority countries, but there are some notable exceptions to this rule, namely Philippines, and several U.S. states. In some jurisdictions, having sexual relations with the king's wife or the wife of his eldest son constitutes treason.