"One Sweet Day" is a song recorded by American singer Mariah Carey and quartet Boyz II Men and written by Carey, Walter Afanasieff and Boyz II Men members Wanya Morris, Shawn Stockman, Nathan Morris, and Michael McCary, and was produced by Carey and Afanasieff for Carey's sixth album Daydream, and was released as the album's second single in 1995.

It holds the record for the longest run at number 1 on the U.S. Billboard Hot 100 (16 weeks total), and is one of the biggest American hits of both Carey's and Boyz II Men's careers.

The song ranked at number 29 on Billboard's Hot 100 All-Time Top Songs.

"One Sweet Day" was inspired by the death of producer and C+C Music Factory member David Cole, as well as Carey's "guitar legend," Def Leppard guitarist Steve Clark, in 1991. While brainstorming with Boyz II Men on a possible collaboration, Carey discovered that the group had written a tribute to their road manager Kahlil Roundtree, who was murdered while they were on tour. Her song plus theirs became "One Sweet Day." David Cole worked with Mariah Carey on her two previous albums, including her MTV UnpluggedEP. During this period, she had arranged to record a song with Boyz II Men. This is also one of the songs that Carey uses to showcase the power of her voice on her Daydreamalbum.

In terms of Billboardmagazine charts, the song is the most successful single for both Mariah Carey and Boyz II Men. The protagonists lament the deaths of friends, but say that they will be together again "one sweet day."

The single became Carey's tenth number 1 single on the Billboard Hot 100 and Boyz II Men's fourth, remaining at the peak for a record-breaking, consecutive sixteen weeks from November 26, 1995 to March 16, 1996. Boyz II Men had previously held this record twice, with "End of the Road" (1992) spending thirteen weeks at the top and "I'll Make Love to You" (1994) spending fourteen. (The former song shares this record with Brandy and Monica's "The Boy Is Mine", and the latter song shared its record with Whitney Houston's "I Will Always Love You". Mariah Carey's 2005's "We Belong Together" and Black Eyed Peas's 2009's "I Gotta Feeling" managed to stay at number 1 for fourteen weeks as well. "One Sweet Day" replaced "Exhale (Shoop Shoop)" by Whitney Houston at number 1, and was replaced by Celine Dion's "Because You Loved Me".

The single also debuted at number 1, making Carey the first artist to have more than one number 1 debut, and also the only artist ever to have two consecutive singles debut at number 1. The single spent twenty-six weeks in the top forty, was certified double platinum by the RIAA, and ranked number 2 on the Hot 100 1996 year-end charts.

Despite its success in the United States, it was a hit elsewhere, as previous Carey singles such as "Fantasy", the first release from Daydream. It reached the top 10 in over 13 countries and topped New Zealand's. While it had success across Europe, but failed to match the success of "Fantasy". It was Boyz II Men's biggest success outside the U.S. since "End of the Road", and was later included on their greatest hits release Legacy: The Greatest Hits Collection(2001).

The song became MTV Asia Hitlist's longest number 1 single, topping the charts for a record-breaking eleven weeks.

"One Sweet Day" was nominated for the 1996 Grammy Awards for Record of the Year and Best Pop Collaboration with Vocals, losing both categories. As two of the top nominees at the 1996 Grammy Awards, Carey and Boyz II Men were invited to perform, and sang "One Sweet Day". Carey began the evening happily as she had been nominated for six Grammys that night. "One Sweet Day" was the current number 1 record on the Hot 100 at that time, and the following morning it was announced that it had just broken the record set by Whitney Houston, giving Carey a "consolation prize".

There are no major remixes of the song, but there is a Chuck Thompson-produced remix, "One Sweet Day" (Chucky's remix), which gives the song a slightly more R&B feel. An a cappella version of the song, known as the Sweet a cappella, features slightly different vocal arrangements and a new intro which also serves as a counter melody throughout the song.

During the Mariah Carey themed week of UK TV show X Factor, boyband JLS performed the song, receiving high praise from all 4 judges and Carey herself (during the masterclass).

The single's music video, directed by Larry Jordan, is made up of footage showing Carey and Boyz II Men writing and recording the song in the studio, as it was difficult for both Boyz II Men and Carey to schedule a time to film a video.

This song is referenced in an episode of the MTV show Daria, "The Misery Chick" (number 113). Principal Angela Li says, "can someone get him out of here so we can sing "One Sweet Day"?"

This song was performed by the top 7 contestants of American Idolseason 7. It was the group performance on the elimination night.

Worldwide CD single

Japanese CD maxi-single

UK CD maxi-single #1

UK CD maxi-single #2

U.S. CD maxi-single