"Watermelon Sugar" is a song by English singer Harry Styles from his second studio album Fine Line (2019), included as the album's second track. Styles wrote the song with Mitch Rowland and the song's producers, Tyler Johnson and Kid Harpoon. They were inspired by the American novel "In Watermelon Sugar" (1968) by Richard Brautigan. "Watermelon Sugar" is a horn and guitar-driven rock, funk-pop, and indie pop song.

Originally released as a promotional single from his album "Fine Line" on 16 November 2019, "Watermelon Sugar" was later released as the album's fourth single on 15 May 2020. The song was well received by music critics, with many praising its concept and Styles' vocals. The single also performed well commercially, peaking within the top ten in more than 20 countries, including the United Kingdom, Australia, Canada, and the United States, where it became Styles' first number-one single on the Billboard Hot 100 chart. It was also nominated for a Grammy Award for Best Pop Solo Performance.