Firth of Fifth is a song by the British progressive rock band Genesis. It first appeared as the third track on the 1973 album Selling England by the Pound, and was performed as a live piece either in whole or in part throughout the band's career.

The title is a pun on the estuary of the River Forth in Scotland, commonly known as the Firth of Forth. Though the song is credited to the entire band, most of the music was composed by keyboardist Tony Banks. He had written the bulk of the song by 1972, presenting it as a candidate for the album Foxtrot (1972), but it was rejected. He redesigned the piece, which the group accepted as a candidate for Selling England by the Pound. Banks worked on the lyrics with the group's Mike Rutherford, which he later dismissed, saying they were "one of the worst sets of lyrics [I have] been involved with."

The song starts out with a classical-style grand piano introduction played by Banks. This section is rhythmically complex, with certain bars in the rare time signatures of 13/16 and 15/16, alternating with bars of 2/4. This section changes tempo and segues into the first section of lyrics, accompanied by Phil Collins on drums and a chord progression between the Hammond organ (Banks) and guitar (Steve Hackett). The song then features a flute melody played by Peter Gabriel, followed by a synth-driven instrumental section which restates the opening piano theme. Hackett then plays the flute melody using violin-like guitar tones.Peter Gabriel then sings a brief section of lyrics before Banks concludes the song on piano.

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