Anal sex is a form of human sexual behavior which encompasses many sexual acts involving the anus, anal cavity, sphincter valve and/or rectum. The term anal sex, however, is often restricted to anal intercourse: the insertion of the erect penis into the rectum.

It is a form of sexual behavior considered to be comparatively high in risk, due to the vulnerability of the tissues and the septic nature of the anus. As the rectal mucosa provides little natural lubrication, a lubricant (such as a personal lubricant, for example) is most often required or preferred when penetrating the anus.