"Fire in My Heart" is the tenth single by Welsh rock band the Super Furry Animals. It was the second single to be taken from the group's 1999 album Guerrilla, and reached number 25 in the UK Singles Chart after its release on 9 August 1999. The track, originally titled "Heartburn", has been described by the band's singer Gruff Rhys as a country and western song with lyrics that offer "soul advice".

Critical reaction to "Fire in My Heart" was generally positive with the NME stating that it confirmed the band's position as the best British singles band in "ages and ages" and placing the track at number 25 in their singles of the year chart for 1999. The music video for the song was directed by Jake & Jim and shows Super Furry Animals waiting to leave the planet Mars, having played a concert there.

"Fire in My Heart" was originally called "Heartburn", a name which the group's singer Gruff Rhys felt was more poignant and gave the song "a twist", but other members of the band were not happy with the title so the name was changed. Rhys has described the track as a country and western song which was written with absolute sincerity despite featuring clichéd lyrics. The song is "soul advice" and is about "all kinds of people in your life". The track was recorded in the middle of 1998 at Real World Studios, Box, Wiltshire, along with the rest of Guerrilla, and was produced by the Super Furry Animals.B-side "The Matter of Time" was considered for inclusion on Guerrilla, but the band felt the album would be too self-indulgent if the song were added to the record's track listing. The group instead opted to include the "stupid" song "The Teacher", a decision which Rhys has called a decisive moment in the creation of the record.

"Fire in My Heart" is 2 minutes and 45 seconds long and is in the key of E major. The song begins with Gruff Rhys singing the first verse backed only by finger picked acoustic guitar. Rhys's vocals are joined by sparse drums, a synthesizer, organ and harmony backing vocals in the second verse which immediately follows the first at 31 seconds. The drums become louder during the third and final verse which leads into a bridge that begins after 1 minute and 31 seconds. During the bridge Rhys sings the lines "Oh the monkey puzzle tree has some questions for the watchdogs of the profane, and I ask, is it sad that I'm driving myself mad as this fire in my heart turns blue". A key change to F♯ major follows for the final verse, with Rhys repeating the line "I've got a fire in my heart for you" backed by multiple harmony vocals. The song ends with a coda during which Rhys elongates the word "you" over the chords A♯ minor, A, G♯ minor and F♯.

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