"Abacab" is a song by the British rock band Genesis, released on 14 August 1981. It was produced by Genesis and distributed in the United States by Atlantic Records and Warner Music Group. The song, written by Tony Banks, Phil Collins, and Mike Rutherford, was featured on Genesis' album of the same name. The song was the second single from the album in the US, where it peaked at No. 26 on the Billboard Hot 100 chart in early 1982. It stayed in the Top 40 for 6 weeks. It was also a top 10 hit on the British pop chart, where it peaked at No. 9.

The title is taken from the structure of an early version of the song. Guitarist Mike Rutherford explained in an interview that the band labelled various sections of the song with letters of the alphabet, and at one point the sections were ordered A-B-A-C-A-B. Rutherford commented that the completed song no longer followed this format, but the name was kept nevertheless.

The track was regularly performed on the band's 1981 Abacab tour, the 1982 Three Sides Live Encore Tour, the 1983/84 Mama Tour and the 1986/87 Invisible Touch tours. On the first two tours, Phil Collins would sing the chorus in a high falsetto while Banks and Rutherford sang the lower harmonies. For the later tours, Collins would sing the chorus in a lower octave while Rutherford sang the higher falsetto harmonies. Genesis rehearsed the song for the 2007 reunion, but it was not included in the final setlist. The song has been covered by Umphrey's McGee live.

The song's LP version has an extensive instrumental arrangement as an outro, while the single edit eliminates that entirely and recycles part of the opening arrangement as a quick outro. (The video also uses this arrangement.)

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