"One Sweet Day" is a song by American singer Mariah Carey and R&B group Boyz II Men. The song was written by Carey, Walter Afanasieff and Boyz II Men: Wanya Morris, , Nathan Morris, and Michael McCary. "One Sweet Day" was produced by Carey and Afanasieff for her fifth studio album, Daydream, and was released as the album's second single on November 14, 1995 (1995-11-14). The song speaks about the death of a loved one, how the protagonist took their presence for granted and misses them, and finally about seeing the person in heaven. Both Carey and Boyz II Men wrote the song about specific people in their lives, being inspired by sufferers of the AIDS epidemic, which was globally prevalent at that time.

"One Sweet Day" received widespread acclaim from critics, many of whom praised its lyrical content and vocals, as well as calling it a standout track from Daydream. It was ranked first in Rolling Stone's reader's poll for the Best Collaboration of All Time. The song spent 16 weeks atop the Billboard Hot 100 in the United States, becoming the longest running number one song on the chart. Subsequently, "One Sweet Day" became the Billboard's most successful song of the 1990s, topping the Hot 100 decade-end chart. Internationally, the song topped the charts in Canada and New Zealand, and reached the top-ten in Australia, Belgium, France, Ireland, the Netherlands, Norway, Sweden and the United Kingdom.

Carey performed "One Sweet Day" live alongside Boyz II Men at the 38th Grammy Awards ceremony, held on February 26, 1996. Additionally, the song was performed at Princess Diana's memorial service in September 1997. "One Sweet Day" was part of the set list on several of Carey's succeeding tours, making its debut during the album's accompanying set of concerts, the Daydream World Tour. It is featured on her compilation albums, #1's (1998), Greatest Hits (2001), The Ballads (2008), and #1 to Infinity (2015).

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