"Bad Guy" (stylized in lowercase) is a song by the American singer-songwriter Billie Eilish and the fifth single from her first studio album, When We All Fall Asleep, Where Do We Go? (2019). It was released on March 29, 2019, by Darkroom and Interscope Records. The song was described by media as electropop, dance-pop, and trap-pop with minimalist instrumentation. In the lyrics, Eilish taunts someone for being tough while suggesting that she is a more resilient bad guy than he is. Eilish wrote "Bad Guy" with its producer Finneas O'Connell. Another version of the song, a collaboration with the Canadian singer Justin Bieber, was released on July 11, 2019.

Upon release, "Bad Guy" received mostly positive reviews. It topped the US Billboard Hot 100 and 16 other international charts including Australia, Canada and New Zealand, and peaked at number two on the UK Singles Chart. In the US, it ended the record-breaking 19-week run of "Old Town Road" by Lil Nas X featuring Billy Ray Cyrus. "Bad Guy" has received several certifications, including a tenfold platinum award from the Australian Recording Industry Association (ARIA) and a Diamond certification from the Recording Industry Association of America (RIAA). The song received several awards, including Record and Song of the Year at the 62nd Annual Grammy Awards.

Dave Meyers directed the music video, which depicts Eilish involved in several activities such as wild dancing, suffering a nosebleed and sitting on the back of a man doing push-ups. Reviewers noted the video for its camp elements and eccentric imagery.

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