In 18th- and 19th-century Britain, a molly house or molly-house was a meeting place for homosexual men. The meeting places were generally taverns, public houses, coffeehouses or even private rooms where patrons could either socialise or meet possible sexual partners.

Despite the reputation of molly-houses as places having primarily sexual connotations, rather than as social meeting places, some historians are reluctant to classify them specifically as brothels. Rictor Norton, for example, argues that the regular customers could have been in fact mutual friends, at least at the beginning, since consistent evidence concerning male prostitution seems to be insufficient in Britain until the 1780s.

From 1533 onwards, homosexual relations and sexual activities remained illegal and were frequently prosecuted, with homosexual sexual activities being included in the offence categories of buggery and sodomy (the terms which were often used interchangeably), they remained capital offences until 1861. From the 16th century onwards until 1861, particularly during the 1720s, molly-houses came to be the scenes of raids and arrests, and their customers frequently became targets for blackmail.

Molly-houses can be considered a precursor to some types of contemporary meeting places for the gay community, such as cottaging.

The word molly (also spelt as molley, mollie, mally) is a pet-form of the female forename Mary, and had two main connotations in 18th century English. The first one is close to the word moll, designating a lower-class girl or woman, occasionally a prostitute. The second one is classified as slang, defining an effeminate, usually homosexual, male. Along with the possible perception of intrinsic female features deriving from the association with the name Mary, another possible origin of this denomination for a homosexual man could be found in the Latin form mollis, indicating the supposed passive-effeminate partner in male homosexual relationships. In a 1762 Swedish/English dictionary by Jacob Serenius and in a 1767 French/English dictionary by Thomas Nugent the word was present, but simply defined a sodomite, without effeminate connotations.

Other uses of the word can be seen in the verb to molly (to have homosexual intercourse), in the expressions mollycot (a British regional expression indicating man interested in activities traditionally associated with women) and Miss Molly (referring to an effeminate or homosexual male). The Kent tradition of Hoodening includes the participation of a Molly; a male who takes on womanhood for the night. She is referenced (often humorously) in hoodening songs.

According to one historian, English society of Georgian era accorded high importance to the concepts of family and household as fundamental units for reproduction, subsistence and interaction between generations: in this context, male and female roles evolved into more static forms. Men were associated with an active, assertive role both in sexual behaviour and in managing the household, while women were "defined in terms of their maternal functions", contrarily to a tradition common at the start of the century, attributing them features related to lustfulness and aggressiveness in sexual matters. It is possible to see that the notion of molly-house was rooted in the emergence of a distinctive identity according to gender and sexual orientation, a peculiar social phenomenon considered crucial by some critics in gender studies.
As stated by Robert Shoemaker, "... any activity (such as homosexual intercourse), in which [men] could be seen as acting passively was further marginalised".

Some of the activities popular among the homosexual community, that were seemingly taking place in molly-houses (by nature, marginalised meeting places), were often associated both to female roles and to a family environment (e.g. cross-dressing, "marriage" and "mock birth" rituals, as described in a satire work of the time, Edward Ward's Satyrical Reflections on Clubs, Chapter XXV "Of the Mollies Club").