"I Feel Fine" is a riff-driven rock song written primarily by John Lennon (credited to Lennon/McCartney) and released in 1964 by the Beatles as the A-side of their eighth UK single. The single reached the top of the UK charts on 12 December of that year, displacing The Rolling Stones' "Little Red Rooster," and remained there for five weeks. It also reached the top of the Billboard Hot 100 in late 1964. The B-side was "She's a Woman".

"I Feel Fine" was the first of six number one songs in a row on the American charts, a record at the time. The subsequent singles were "Eight Days a Week", "Ticket to Ride", "Help!", "Yesterday", and "We Can Work It Out". The record was equaled by The Bee Gees in 1979 and surpassed by Whitney Houston in 1988.

Lennon wrote the guitar riff while in the studio recording "Eight Days a Week". "I wrote 'I Feel Fine' around that riff going on in the background," he recalled. "I told them I'd write a song specially for the riff. So they said, 'Yes. You go away and do that,' knowing that we'd almost finished the album Beatles for Sale. Anyway, going into the studio one morning, I said to Ringo, 'I've written this song but it's lousy.' But we tried it, complete with riff, and it sounded like an A side, so we decided to release it just like that." George Harrison said that Lennon's riff was influenced by a riff in "Watch Your Step", a 1961 release written and performed by Bobby Parker and covered by The Beatles in concerts during 1961 and 1962.

Paul McCartney said the drums on "I Feel Fine" were inspired by Ray Charles's "What'd I Say".

At the time of the song's recording the Beatles, having mastered the studio basics, had begun to explore new sources of inspiration in noises previously eliminated as mistakes (electronic goofs, twisted tapes, talkback). "I Feel Fine" marks the earliest example of the use of feedback as a recording effect. Artists such as Jimi Hendrix, The Kinks, and The Who used feedback, but Lennon remained proud of the fact that the Beatles were the first group to actually put it on vinyl.

The intro to "I Feel Fine" starts with a single, percussive (yet pure-sounding) note (a high "A" harmonic) played on McCartney's Hofner bass guitar that sustains, perhaps beyond any song previously recorded. It is then transformed and distorted via feedback. According to McCartney, "John had a semi-acoustic Gibson guitar. It had a pick-up on it so it could be amplified... We were just about to walk away to listen to a take when John leaned his guitar against the amp. I can still see him doing it... it went, 'Nnnnnnwahhhhh!" And we went, 'What's that? Voodoo!' 'No, it's feedback.' Wow, it's a great sound!' George Martin was there so we said, 'Can we have that on the record?' 'Well, I suppose we could, we could edit it on the front.' It was a found object, an accident caused by leaning the guitar against the amp."

While sounding very much like an electric guitar, Lennon played it on an acoustic (a Gibson model J-160E), employing the guitar's onboard pickup and 1960s sound effect devices to make the acoustic guitar sound more electronic. The intro riff around a D major chord progresses to a C, then a G, where the G major vocals begin. Just before the coda, Lennon's intro riff (or ostinato), is repeated with a bright sound by George Harrison on electric guitar (a Gretsch Tennessean), followed by the more electricsound of John on amped acoustic.

In the US, the song was released on their Capitol album Beatles '65, and is presented in a duophonic mix featuring a layer of reverb added by executive Dave Dexter, Jr..

In the UK, the song was released on the LP format on A Collection of Beatles Oldies. A true stereo version can be found on the Past Masters Vol 1and Beatles 1CDs.

There is also another stereo version that sounds the same, but with whispering at the very beginning which appears on the original release of 1962–1966.