Promiscuity is the practice of engaging in sexual activity frequently with different partners or being indiscriminate in the choice of sexual partners. The term can carry a moral judgment. A common example of behavior viewed as promiscuous by many cultures is the one-night stand, and its frequency is used by researchers as a marker for promiscuity.

What sexual behavior is considered promiscuous varies between cultures, as does the prevalence of promiscuity. Different standards are often applied to different genders and civil statutes. Feminists have traditionally argued a significant double standard exists between how men and women are judged for promiscuity. Historically, stereotypes of the promiscuous woman have tended to be pejorative, such as "the slut" or "the harlot", while male stereotypes have been more varied, some expressing approval, such as "the stud" or "the player", while others imply societal deviance, such as "the womanizer" or "the philanderer". A scientific study published in 2005 found that promiscuous men and women are both prone to derogatory judgment. Kennair et al. (2023) found no signs on a sexual double standard in short-term or long-term mating contexts, nor in choosing a friend, except that women's self-stimulation was more acceptable than men's.

Promiscuity is common in many animal species. Some species have promiscuous mating systems, ranging from polyandry and polygyny to mating systems with no stable relationships where mating between two individuals is a one-time event. Many species form stable pair bonds, but still mate with other individuals outside the pair. In biology, incidents of promiscuity in species that form pair bonds are usually called extra-pair copulations.