Mammary intercourse describes a sex act, performed as foreplay or as non-penetrative sex, that involves the stimulation of the male penis by the female breasts. Commonly, this sex act involves the man placing his penis in the woman's cleavage and thrusting between her breasts, while the breasts are squeezed around the penis for additional stimulation.

Mammary intercourse can also be combined with fellatio. Researcher Austen Woods, in a study of the condom usage habits of New Zealand's sex workers, said that they offered safe sex alternatives to clients who refused to wear a condom.

Woods quoted a sex worker saying that mammary intercourse was one alternative used, because she claimed it felt to clients like sex without a condom when performed by a woman with large breasts.

It should be noted however that mammary intercourse is not truly safe sex, as some STDs can be transmitted by skin-to-skin contact. When mammary intercourse continues to the point of ejaculation, the semen can land on the breasts, face, or in the mouth of the woman. A slang term for the appearance of semen landing on the woman's upper chest, neck, or shoulders is a "pearl necklace".

Several slang terms refer to mammary intercourse, such as boobjob, titfuck or titty fuck in the United States, as well as tit wank or French fuck in the United Kingdom. The Japanese sex industry uses the term paizuri (from oppai, a slang word for breast).