"XO" is a song by the American singer Beyoncé from her fifth studio album, Beyoncé (2013). Columbia Records released it to US pop radio on December 16, 2013. Beyoncé, Ryan Tedder, and Terius "The-Dream" Nash wrote and produced "XO", a mid-tempo pop and electro-rock ballad that is instrumented by synthesizers and drums. It incorporates elements of hip-hop and dancehall and has a call and response chorus. A love song, its lyrics detail a narrator's plea for a lasting and stable romance amidst the ups and downs of life.

Music critics generally acclaimed the radio- and stadium-friendly production and the intimate lyricism of "XO". The track's beginning contains an audio sample from the Space Shuttle Challenger disaster, which was criticized by the families of the lost crew and NASA. Beyoncé released a statement saying that the sample was meant to be a tribute to the Challenger crew. "XO" charted and received platinum certifications in Australia, Canada, New Zealand, and the UK. In the US, the single peaked at number 45 on the Billboard Hot 100 and has been certified double platinum.

Terry Richardson directed the music video for "XO", filmed in Coney Island in late August 2013. It was made available on iTunes on December 13, 2013 and released online three days later. On the Mrs. Carter Show World Tour, Beyoncé performed the song during the last stops of the North American leg in December 2013 and the second European leg in early 2014. She also performed the song at the Brit Awards 2014 and the 2014 MTV Video Music Awards. In 2014, Haim covered the song during their several concerts, and John Mayer released his cover version as a single.

"XO" was written by Beyoncé, Terius "The-Dream" Nash and Ryan Tedder while the production was helmed by the aforementioned group as well as Chauncey "Hit-Boy" Hollis and HazeBanga. Ramon Rivas engineered the song, with assistance from Justin Hergett. Beyoncé's vocals were recorded by Stuart White and Bart Schoudel while all instrumentation and programming was carried out by Tedder. The song was finally mixed by Andrew Scheps. "XO" was recorded in four studios: Jungle City Studios and Oven Studios, both in New York City, Trackdown Studios in Sydney and Tritonus Studios in Berlin.

During an interview with Australian radio station Nova in November 2013, Tedder revealed that he had collaborated with Beyoncé on a song for her then-upcoming album along with The-Dream, further saying, "Personally the song we did I like more than 'Halo'. I think it's a bigger, better song". However, he acknowledged that he did not know when the singer planned to release new material for her fifth album. The previous month, it was reported by a source to the Daily News that Beyoncé planned to release a new single and video on December 3, 2013. The publication further speculated that the single was expected to be the song whose music video was filmed in August of the same year – "XO". Later, when Beyoncé was released, Billboard reported that "Blow" would be released as the contemporary hit radio single both in the US and worldwide, and "Drunk in Love" would be sent to urban radio in the US only; "XO" was scheduled to be released as the second worldwide radio single in 2014. However, radio programmers reportedly pushed back against the release of "Blow" to mainstream stations, finding its lyrics to be very explicit for the format and proposing that "XO" be released in its place. The release of "Blow" was therefore scrapped and "XO" impacted contemporary hit radio in Italy and adult contemporary radio in the US on December 16, 2013. It was also sent to US mainstream, urban and rhythmic radio on December 17. A digital single, with "Drunk in Love" as a double A-side, was released to streaming services such as Spotify, where the full Beyoncé album was not yet available to stream.

"XO" is a midtempo pop and electro-rock love power ballad that strongly resembles Beyoncé's own 2008 song "Halo", also produced by Tedder, and contains influences of the electronic rock and reggaeton genres. The sheet music for "XO" was published in common time in the key of C major, with a tempo of 85 beats per minute. It carries a "celebratory, bumping Caribbean" groove and a marching-drum beat with shuffling dancehall influences. The instrumentation includes jittery keyboards, synthesizers, percussion and electronic musical instruments. The song also consists of electronic flourishes and looped riff patterns brought about by an organ instrument. "XO" was also noted for departing from the album's minimalistic sexual nature, due to its pure pop sound. Jordan Sargent of Complex magazine wrote that "XO" contains influences of Jamaican singer Tanya Stephens. Chris Bosman from Consequence of Sound described the song as a blend of "cinematic reach of modern Top 40 pop with the patience and melancholy of post-808s & Heartbreaks [sic] hip-hop." Kevin Fallon from The Daily Beast compared the song's stadium sound to tracks by the band U2.

Beyoncé's vocals in the song span from the note of A₃ to the note of C₅. When asked about her raw vocals in the song previously unheard on her material and the song's less polished sound, Beyoncé revealed that "XO" was recorded when she had a bad sinus infection and that it was recorded as a demo in several minutes. The original demo vocals were kept for a year without being re-recorded as the singer "really loved the imperfections" and wanted to focus on the album's music instead of the vocals.

The lyrics of "XO" cheerfully celebrate love and life as Beyoncé attempts "to create light from darkness". It has been described as a universal love song which describes different types of relationships. The singer emphasizes how important living in the present with a person's loved one is as life is unpredictable, as well as how both tragic and magnificent life can be. Throughout the song, the singer adopts a hopeful attitude and seems to have gone in trance with love, a state further augmented by her usage of a generally low register. Caitlin White from the website The 405 interpreted the song's lyrics in depth: