"The Rain Song" is a ballad of over seven minutes in length. Guitarist Jimmy Page originally constructed the melody of this song at his home in Plumpton, England, where he had recently installed a studio mixing console. A new Vista model, it was partly made up from the Pye Mobile Studio which had been used to record the group's 1970 Royal Albert Hall performance and The Who's Live at Leeds album.

With a working title of "Slush", a reference to its easy listening simulated orchestral arrangement, Page was able to bring in a completed arrangement of the melody, for which singer Robert Plant composed the words. Plant ranks his vocal performance on the track as one of his best. The song also features a Mellotron played by John Paul Jones to add to the orchestral effect, while Page plays a Danelectro guitar.

George Harrison was reportedly the inspiration for "The Rain Song" when he made a comment to Led Zeppelin drummer John Bonham, about the fact that the group never wrote any ballads. In Light and Shade: Conversations with Jimmy Page, biographer Brad Tolinski quotes Page:

George [Harrison] was talking to Bonzo one evening and said, 'The problem with you guys is that you never do ballads.' I said, 'I'll give him a ballad,' and I wrote 'Rain Song,' which appears on House of the Holy. In fact, you'll notice I even quote 'Something' in the song's first two chords.

However, there is some disagreement about exactly what was said by whom.

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