"Hello" is a song recorded by British singer-songwriter Adele, released on 23 October 2015 by XL Recordings as the lead single from her third studio album, 25 (2015). Written by Adele and the album's producer, Greg Kurstin, "Hello" is a piano ballad with soul influences (including guitar) and lyrics that discuss themes of nostalgia and regret. Upon release, the song garnered critical acclaim, with reviewers comparing it favourably to Adele's previous works and praised its lyrics, production and Adele's vocal performance. It was recorded in Metropolis Studios, London.

"Hello" was a massive global success, topping the records charts in a record-setting 30 countries, including in the United Kingdom where it became Adele's second chart topper, following "Someone Like You", and had the largest opening week sales in three years. In the United States, "Hello" debuted atop the Billboard Hot 100, reigning for ten consecutive weeks while becoming Adele's fourth number-one single on the chart and breaking several records, including becoming the first song to sell over a million digital copies in a week. By the end of 2015, it had sold 12.3 million units globally (combined sales and track-equivalent streams) and was the year's 7th best-selling single while it stands as one of the best-selling digital singles of all-time.

The accompanying music video was directed by Xavier Dolan and co-stars Adele and Tristan Wilds. It broke the Vevo and YouTube records for achieving over 27.7 million views within a 24-hour span, and the record for the shortest time to reach one billion views (87 days) respectively. "Hello" broke the iTunes records for being number 1 in most countries (102 countries). The music video for the song received seven nominations at the 2016 MTV Video Music Awards, including Video of the Year and Best Female Video. At the 59th Annual Grammy Awards, "Hello" won three awards: Record of the Year, Song of the Year, and Best Pop Solo Performance. The song also won the Brit Award for British Single, and APRA Award for International Work of the Year.

"Hello" was written by Adele and Greg Kurstin and produced by Kurstin, who also played bass, guitar, drums, piano and keyboards, while Adele played drums. "Hello" was written in Chiswick, London, something not normally done by Adele, who said she likes to write her music at home. The writing process for the song was slow, taking six months to complete. Initially Adele and Kurstin started writing the first verse; finishing half of the song, six months later Adele contacted Kurstin to finish the song with her, with Kurstin stating he was not sure "if Adele was ever going to come back and finish it."

"Hello" is a soul piano ballad, played in the key of F minor at a tempo of 79 beats per minute. The repeated chord progression heard in the verse, played by the piano, follows two progressions and a passage (Bridge) progression (passage or Bridge progression connects two main progressions to each other) of Progression #1: Fm(i)–A♭(III)–E♭(VII)–D♭(VI), Passage progression: Fm(i)–E♭(VII)-Cm(v)-D♭(VI)-Fm7(i7)-E♭(VII)–D♭(VI), Progression #2:Fm(i)–D♭/F(VI)–A♭(III)–E♭(VII). According to Musicnotes.com, Adele's vocals span from F₃ to A♭₅ in the song. During the chorus, Adele is heard singing the lines over layers of backing vocals, piano and drums which were described by The Daily Telegraph as having "a very luscious wall of sound".

Lyrically, the song focuses on themes of nostalgia and regret, and was seen as a follow-up to her single "Someone like You" appearing to reflect on a failed relationship. The song's lyrics were also seen as being conversational, revolving around "all the relationships of her past", ranging from friends, family members and ex-partners. Speaking on the song's lyrical content, Adele told Nick Grimshaw on The Radio 1 Breakfast Show: "I felt all of us were moving on, and it's not about an ex-relationship, a love relationship, it's about my relationship with everyone that I love. It's not that we have fallen out, we've all got our lives going on and I needed to write that song so they would all hear it, because I'm not in touch with them." According to Adele, the line "Hello from the other side" signifies "the other side of becoming an adult, making it out alive from your late teens, early twenties."

On 18 October 2015, a 30-second clip of "Hello" was played during a commercial break on The X Factor in the United Kingdom. The commercial teased what was then new material, with her vocals accompanied by lyrics on a black screen. Josh Duboff of Vanity Fair wrote that "the Internet collectively lost their minds" after the broadcast of the trailer. On 22 October, Adele announced the upcoming release of 25 to her fans on Twitter. She also shared that "Hello" would be released on 23 October as the lead single off of the album. On 23 October, Adele joined Nick Grimshaw's show on BBC Radio 1 for the song's premiere.

"Hello" received universal critical acclaim. Alexis Petridis of The Guardian described it as "a big ballad, but a superior example of its kind", and opined that the song is "precisely the kind of lovelorn epic ballad that made Adele one of the biggest stars in the world." Writing for The Independent, Emily Jupp stated in her review of the song that it "might not be groundbreaking, but Adele's return with her familiar, smoky sound is very welcome". She called it an "'if it ain't broke' ballad" and said: "Adele does what she does best, belting out emotional tales of love and loss much the same as with her last album, 21, but this time, with a little more self-forgiveness."