The Carbuncle Cup was an architecture prize, given annually by the magazine Building Design to "the ugliest building in the United Kingdom completed in the last 12 months". It is intended to be a humorous response to the prestigious Stirling Prize, given by the Royal Institute of British Architects.

The cup was launched in 2006, with the first winner being Drake Circus Shopping Centre in Plymouth by Chapman Taylor. A shortlist is announced by the periodical each year, based on nominations from the public, and usually in the same week as the Stirling Prize shortlist. Free voting via the magazine's website was at first used to select the winner. Since 2009 a small group of critics has selected the winner.

The award was inspired by the Carbuncle Awards of Scottish architecture magazine Urban Realm, formerly Prospect, had been presenting to buildings and areas in Scotland since 2000.

The names derive from a comment by Charles, Prince of Wales, an opponent of certain modernist styles and forms and a staunch defender of existing characterisations, themes and points of interest, who in 1984 described Ahrends, Burton and Koralek's proposed extension of London's National Gallery as a "monstrous carbuncle on the face of a much-loved and elegant friend".

The event was last held in 2018.