The R18 certificate represents a film or video classification given by the British Board of Film Classification (BBFC). It is intended to provide a classification for works that are within British obscenity laws, but exceed what the BBFC considers acceptable for its 18 certificate. In practice, this means hardcore pornography.

Under the terms of the 1984 Video Recordings Act all videos sold or distributed within the UK must either be given a certificate by the BBFC, or ruled exempt by them. Uncertificated recordings are not legal, regardless of content. The R18 certificate is the most restrictive of the certificates, and videos given this certificate may only be shown in licensed cinemas, or sold direct to the buyer in person in licensed sex shops, and not by mail order or other remote means such as by telephone or over the Internet. According to the BBFC there are currently around 250 such licensed shops.

Since the boundary of British obscenity law is vague, the BBFC deliberately errs on the side of caution . It specifies in detail what kinds of acts are permitted to be depicted in works receiving an R18 certificate, and which are not. In particular, it prohibits the depiction of acts which are illegal, degrading, violent or non-consensual. However, apart from these restrictions, it allows the depiction of most sex acts, including vaginal sexual intercourse, oral sex, masturbation, and anal sex, between any combination of men and women. It also has recently allowed the depiction of visibly consensual sexual bondage and very mild BDSM activities.

There has been some controversy about the prohibition of the depiction of female ejaculation, which the BBFC considers to be the same as urination during sex, the depiction of which it regards as "degrading or dehumanising". As of 2005, the BBFC still appears to regard the depiction of urination on oneself or others, "urination accompanied by sexual activity", or "immediately before/after", sexual activity, or with licking of urine, as prohibited by the Obscene Publications Act 1959. Nevertheless, the BBFC appears to have R18-rated a number of videos based on the theme of urination (presumably without any of the acts referred to above).

As of 2004, the broadcasting of R18 material is prohibited, even on encrypted digital channels.