"Birthday" is a disco song recorded by American singer Katy Perry for her fourth studio album, Prism (2013). She co-wrote the song with Bonnie McKee and its producers Dr. Luke, Max Martin, and Cirkut. Critics, as well as Perry herself, have compared the track to works by Prince and Mariah Carey. Through double entendres in the lyrics of "Birthday", Perry makes sexual references while celebrating a partner's birthday. Capitol Records sent the track to mainstream and rhythmic radio on April 21, 2014 as the album's fourth single.

Following the release of Prism, "Birthday" entered the single charts of South Korea and France. After being released as an official single, it reached the top 30 in Australia, UK, and Netherlands, number 17 on the US Billboard Hot 100, and number 7 on the Canadian Hot 100. A music video for the track was released on April 24, 2014. Primarily recorded with hidden cameras, it features Perry disguised as five different characters in birthday parties and other celebrations. The makeup effects disguises for Perry were designed and created by Tony Gardner. Jess Glynne covered the song on BBC Radio 1's Live Lounge, and Halsey on SiriusXM.

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