Group sex is sexual activity involving more than two people. Participants in group sex can be of any sexual orientation or gender. Any form of sexual activity can be adopted to involve more than two participants, but some forms have their own names.

Group sex most commonly takes place in a private sex party or semi-public swinger gathering, but may also take place at massage parlors or brothels or, in some jurisdictions, at purpose-built locations such as sex clubs. In places where non-monogamous sex is taboo or illegal, group sex may take place in private or clandestine locations including homes, hotel rooms, or private clubs.

Fantasies of group sex are extremely common among both men and women. Many forms of sexual behavior were reported by Alfred Kinsey's subjects, but the official Kinsey Reports website does not mention threesomes or group sex in the summary of Kinsey's findings.

Group sex is a popular subgenre in pornographic films.

In principle, any sexual behavior performed by more than two people can be referred to as group sex, but various terms are used to describe particular acts or combinations of people. Many swingers argue that non-swingers have conflated the terms because of lack of understanding and that there are distinct differences among the terms with specific meanings as to number, intent, sexual orientation, and familiarity of the persons involved.

A sex party is a gathering at which sexual activity takes place. There are a number of types of sex parties.

An orgy is a gathering where guests freely engage in open and unrestrained sexual activity or group sex; and a bunga bunga orgy is an orgy in which participants have sex underwater, such as in a swimming pool or a hot tub.

A swinger party or partner-swapping party is a gathering at which individuals or couples in a committed relationship can engage in sexual activities with others as a recreational or social activity.