"Aqualung" is a song by the British progressive rock band Jethro Tull, and the title track from their Aqualung (1971) album. The song was written by the band's frontman, Ian Anderson, and his then-wife Jennie Franks.

While this track was never a single, its eponymous album Aqualung was Jethro Tull's first American Top 10 album, reaching number seven in June 1971.

The lyrics conveys a story of a homeless man named Aqualung. In verses 1 and 2, he is shown as a dirty tramp, who can't but evoke a sense of hopelessness and disgust, as not a single person in the world would ever help him. Aqualung is lone and sick, doomed to "bend to pick a dog-end" (British slang for a discarded cigarette butt). In verse 3, he sees a light of hope for compassion, arising in encounter of the very storyteller (Aqualung my friend, don't you start away uneasy). But still, in verse 4 the inevitable seems to happen, and a poor creature is "snatching <his> rattling last breaths". The indifference to grief is nevertheless reigning on earth, "and the flowers bloom like madness in the spring".

The lyrics compare the tramp's unhealthy breathing to "deep sea diver sounds", referring to the actual aqualung device.

The original recording runs for 6:34. In an interview with singer Ian Anderson in the September 1999 Guitar World, he said:

Aqualung wasn't a concept album, although a lot of people thought so. The idea came about from a photograph my wife at the time took of a tramp in London. I had feelings of guilt about the homeless, as well as fear and insecurity with people like that who seem a little scary. And I suppose all of that was combined with a slightly romanticized picture of the person who is homeless but yet a free spirit, who either won't or can't join in society's prescribed formats.

So from that photograph and those sentiments, I began writing the words to "Aqualung". I can remember sitting in a hotel room in L.A., working out the chord structure for the verses. It's quite a tortured tangle of chords, but it was meant to really drag you here and there and then set you down into the more gentle acoustic section of the song.

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