"Speed of Sound" is a song by British rock band Coldplay. It was released as the lead single for their third studio album, X&Y (2005), on 19 April 2005, following its radio premiere on the previous day at Steve Lamacq's BBC Radio 1 show. Written by all members of the band, the song was built around a piano and guitar riff, peaking into a synthesiser-heavy chorus. A physical version of the single was issued on 23 May in the United Kingdom, containing the B-sides "Things I Don't Understand" and "Proof".

Vocalist and pianist Chris Martin stated that the song was written after the band had listened to English singer Kate Bush. Upon release, it charted in the UK Singles Chart at number two. In the United States, it debuted at number eight on the Billboard Hot 100, becoming their first top ten hit and highest-peaking song until "Viva la Vida" reached number one in 2008.

"Speed of Sound" was named the Song of the Year by the American Society of Composers, Authors and Publishers (ASCAP) and later nominated for Best Rock Song and Best Rock Performance by a Duo or Group with Vocal at the 48th Annual Grammy Awards. It won the Brit Award for Best British Single at the 2006 Brit Awards. In the same year, the music video received four nominations at the MTV Video Music Awards. "Speed of Sound" is also notable for being the billionth download purchased on the iTunes Store.

In an interview, Coldplay vocalist Chris Martin revealed that "Speed of Sound" was written in mid-2004 and was inspired by Martin's daughter, Apple, and English alternative rock singer Kate Bush: "That's a song where we were listening to a lot of Kate Bush last summer, and we wanted a song which had a lot of tom-toms in it. I just had my daughter up also, and was kind of feeling in a sense of awe and wonderment, so the song is kind of a Kate Bush song about miracles." The drumbeat of the song was inspired by Bush's 1985 song "Running Up that Hill". In a separate interview, bassist Guy Berryman said: "We were really trying to recreate the drums on that song for this song, and the chords. Some bands are reluctant to admit that they take things from other artists and bands that they listen to and we're shameless in that respect, we don't mind telling."

The band's opinion towards the song, however, has soured. Martin has said that he dislikes "Speed of Sound", saying in a 2011 interview, "We never got it right." Due to this, Martin is reluctant to perform the song live, citing his feelings towards the song, noting that during an interview with Howard Stern that "an audience can pick up real fast if something is clearly missing."

"Speed of Sound" is a piano-based song with an upbeat tempo. The song begins with the insistently ornate keyboard melody, with a backing synthesizer that is heard throughout the song. The melody then transitions into the first verse being sung by lead singer Chris Martin. It repeats, with the song then transitioning into its second verse. The verses of song feature the rest of the band as the song continues to build. It then climaxes with the anthemic chorus, which features a huge roar of instrumentation that includes electric and bass guitars, drums, and synthesizers. The song then ends with the chorus and its grandiose sound. "Speed of Sound" is written in the key of A Mixolydian.^{[citation needed]}

The lyrics in the song are cryptic. The ending lines of the chorus emphasise belief and assurance: "If you could see it then you'd understand/Ah, when you see it then you'll understand". This belief is restated in the third verse: "Some things you have to believe/others are puzzles puzzling me". The fourth line in the second verse alludes to discovering one's place in the world: "How long am I gonna stand/with my head stuck under the sand".

In his review of X&Y, Bill White of the Seattle Post-Intelligencer newspaper noted that the "slowing ... creative juices" evident in "Speed of Sound" compare to "melodies from both Jeff Buckley ('Last Goodbye') and Keane ('Everybody's Changing')".