"I'm Down" is a song by the English rock band the Beatles, written by Paul McCartney and credited to Lennon–McCartney. It was released on a non-album single as the B-side to "Help!" in July 1965. The song originated in McCartney's attempt to write a song in the style of Little Richard, whose song "Long Tall Sally" the band regularly covered.

Inspired by 1950s R&B and rock and roll numbers, the song's lyrics sing of an unrequited love, but rather than a lament are instead performed in a hysterical, "celebratory frenzy" of self assuredness. Some commentators interpret the song's tone as partially parodic. Melodically uncomplicated, the composition uses only three basic chords. The Beatles recorded "I'm Down" during sessions for their album Help! in June 1965. The first song by the band to incorporate a Vox Continental electric organ, John Lennon plays the instrument in the style of Jerry Lee Lewis, doing glissandos with his elbow. In the decade following its release, the song became a comparative rarity among the band's recordings. It has subsequently appeared on compilation albums such as Rock 'n' Roll Music; the UK edition of Rarities; Past Masters, Volume One and Mono Masters.

"I'm Down" has received praise from several music critics and musicologists, with several mentioning McCartney's strong vocal and the band's raucous performance. The Beatles regularly performed the song during their 1965 and 1966 tours as the closing number, including an especially chaotic performance in August 1965 depicted in the documentary The Beatles at Shea Stadium. Beastie Boys and Aerosmith are among the artists that have covered the song.

I could do Little Richard's voice, which is a wild, hoarse, screaming thing, it's like an out-of-body experience. You have to leave your current sensibilities and go about a foot above your head to sing it. ... A lot of people were fans of Little Richard so I used to sing his stuff but there came a point when I wanted one of my own, so I wrote "I'm Down".

In November 1963, Paul McCartney moved into the family home of his girlfriend, Jane Asher, located at 57 Wimpole Street in central London. He later recalled writing "I'm Down" in the family music room in the basement of the house. Written in the style of Little Richard, the song began as an attempt to replace "Twist and Shout" and "Long Tall Sally" as the closing number of the Beatles' concert tour set list. In an October 1964 interview, McCartney explained that he and John Lennon had been trying for years to write a song like "Long Tall Sally", and that the closest they had come was with their song "I Saw Her Standing There". Comparing the writing process of Little Richard-like songs to abstract painting, he further explained: "[p]eople think of 'Long Tall Sally' and say it sounds so easy to write. But it's the most difficult thing we've attempted. Writing a three-chord song that's clever is not easy". In his authorised biography, Many Years From Now, McCartney remembers "I'm Down" as entirely his composition, but raises the possibility that Lennon added a few lyrics or made minor suggestions in the writing process. In a 1972 interview, Lennon credits the song to only McCartney, but in his 1980 Playboy interview he instead suggests he provided "a little help". Musicologist Walter Everett argues that McCartney often forgetting the song's lyrics in concerts suggests he wrote the song quickly and with little practice.

"I'm Down" is in the key of G major and is in 4/4 (common time). A simple twelve-bar blues number extended into fourteen-bars, the song uses only the chords I, IV and V. One of the few Beatles songs to feature a simple verse form, musicologist Alan W. Pollack suggests that, in the context of the Beatles' 1965 compositions, its simple format is stylistically regressive. The song opens with a solo vocal from McCartney, which music critic Tim Riley sees as the part of the song bearing the most resemblance to "Long Tall Sally", with "one mad voice screaming at the top of its lungs". With neither bass nor drums to clarify the key or downbeat, Pollack writes that "no matter how many times you've heard the song", McCartney's opening vocal is "an effect which retains the power to startle". The repeating refrains incorporate improvisational scat singing and, according to Pollack, get "successively wilder and less structured" with each repeating. Everett writes the concluding coda serves the purpose of "[raising] the rock-and-roll spirit to a higher level of excitement than does the song proper".

The song's lyrics tell the story from the perspective of a pained lover who is frustrated due to an unrequited love. Rather than a lament, the music functions as a "celebratory frenzy" of self assuredness. Pollack writes the song's style originates in a 1950s R&B cliché, being "a semi-improvisatory rave-up" where the lyrics are unimportant compared to the tone in which they are sung. Riley describes the song as an instance of "dancing on your problems", as heard in rock and roll oldies like "That's All Right" and "Blue Suede Shoes". Author Ian MacDonald suggests that, besides being a blues send-up, the lyrics are "a tongue-in-cheek response to Lennon's anguished self-exposure in 'Help!'", opining that the song's "pseudo-hysterics" began as a joke. Riley similarly describes the song as partially parodic, singling out the backing vocals' response of "I'm really down".

The Beatles recorded "I'm Down" on 14 June 1965 during a session for their fifth album, Help!, in which they also recorded McCartney's songs "I've Just Seen a Face" and "Yesterday". Recording in EMI's Studio Two, George Martin produced the session, assisted by balance engineer Norman Smith. The song's basic track features McCartney singing and playing bass, George Harrison on electric guitar and Ringo Starr on drums. On the band's first take, the song did not yet have a definitive ending, McCartney telling Harrison and Starr after the last chorus to "keep it going". The final attempt – take seven – was marked "best".