"Sound of the Underground" is the debut single by British-Irish pop group Girls Aloud, and later featured on their debut album of the same title. The song was written by Miranda Cooper, Brian Higgins and Niara Scarlett, and produced by Higgins and his production team Xenomania. Following Girls Aloud's formation on the ITV1 reality television show Popstars: The Rivals, "Sound of the Underground" was released 16 days later, on 16 December 2002. Commercially, it was an immediate success; it became the year's Christmas number one in the UK, spending four consecutive weeks atop the chart. It also reached number one in Ireland and peaked within the top forty in Australia, Belgium, the Netherlands, Sweden and Switzerland.

The music video was recorded in an empty London warehouse days after Girls Aloud were formed. The song has been performed by Girls Aloud in a number of live appearances including Popstars: The Rivals and each of Girls Aloud's concert tours. Critically appreciated for its juxtaposition of surf guitar against electronic beats, "Sound of the Underground" was praised for its quality for both a girl group and a reality television act. The song was referred to as a defining moment of the decade musically, credited with reshaping British pop for the 2000s.

"Sound of the Underground" was written by Brian Higgins, Niara Scarlett and Miranda Cooper. In an interview with The Daily Telegraph's Alice Vincent, Cooper explained that she was into drum and bass at the time and had been inspired by Josh Abrahams and Amiel Daemion's 1998 single "Addicted to Bass", as well as the popular nursery rhyme "The Wheels on the Bus". The song has been described as "a mechanistic sashay of twangy surf guitar and sultry gang vocals – Girls Aloud explodes like a five-headed Kylie Minogue." The song is written in E minor (recorded in E-flat minor) with a time signature in common time and a tempo of 163 beats per minute. The vocal range spans from G₃ to C#₅. Following typical verse-chorus form, the song consists of a verse followed by a bridge and chorus. The second half of the bridge is also employed as a middle 8.

"Sound of the Underground" was one of sixty songs that Higgins and Cooper had written with the aim of launching their own girl group. The song was originally recorded in 2001 by Orchid, a London girl group that included singer and actress Louise Griffiths and who disbanded before gaining a firm record deal. Irish artist Samantha Mumba was due to record this as her comeback song in 2002 but she opted for "I'm Right Here" instead. It was chosen by Girls Aloud's manager Louis Walsh as their debut single. The finalists of the ITV1 programme Popstars: The Rivals recorded and rehearsed the song a week before the finals.

In the liner essay for the 20th anniversary re-release of the group's debut album, Sound of the Underground, Kimberley Walsh wrote, "I was just glad [the song] wasn't an insipid ballad or something really cheesy – it was definitely a song with a cooler vibe." Nicola Roberts admitted that she was initially anxious about the drum and bass sound of the track, which made it stand out from other chart hits of that era: "We didn't have drum and bass up north at the time; we just had commercial radio ... So I was just very aware that Sound of the Underground sounded different from the pop groups that came before us: Atomic Kitten, Spice Girls, Hear'Say."

Girls Aloud were formed through the show by a public vote on 30 November 2002. The concept of the programme was to produce a boyband and a girlband who would be "rivals" and compete for the Christmas number one single in 2002. Girls Aloud competed against One True Voice, managed by music producer Pete Waterman. Both groups were sent on huge promotional tour. They used a combative "Buy girls, bye boys" slogan to persuade the public to buy their single. A cover of One True Voice's single "Sacred Trust" appears on the special edition bonus disc of Girls Aloud's greatest hits album The Sound of Girls Aloud, alongside an extended performance version of "Sound of the Underground" which includes an instrumental breakdown.

The single was released on two different CD single formats and on cassette. The first CD included a cover of East 17's "Stay Another Day" and Brian Higgins' remix of "Sound of the Underground", while the second CD included the instrumental of "Stay Another Day" and an interview with the girls. In some countries, a Flip & Fill remix of the single was featured.

Pete Waterman caused a media frenzy after accusing Girls Aloud of being unoriginal and not singing on "Sound of the Underground" (after they claimed it was better to release a new song than a cover as the A-side). Many articles falsely claim that this track was then "stolen" from Orchid by Girls Aloud. The track was actually owned by Xenomania (the producers) and the original version was used as a "session singers" version, making it Girls Aloud's and not a cover as some articles suggested. Due to the press attention, the members of Orchid are credited on the single release and remain backing singers on the single. Sarah Harding explained that this just pads out the track and is not uncommon in the industry.^{[citation needed]}