"Instant Karma!" (also titled "Instant Karma! (We All Shine On)") is a song by English musician John Lennon, released as a single on Apple Records in February 1970. The lyrics focus on a concept in which the consequences of one's actions are immediate rather than borne out over a lifetime. The single was credited to "Lennon/Ono with the Plastic Ono Band", apart from in the US, where the credit was "John Ono Lennon". The song reached the top five in the British and American charts, competing with the Beatles' "Let It Be" in the US, where it became the first solo single by a member of the band to sell a million copies.

"Instant Karma!" was conceived, written, recorded, and released within a period of ten days, making it one of the fastest-released songs in pop music history. The recording was produced by Phil Spector, marking a comeback for the American producer after his self-imposed retirement in 1966, and leading to him being offered the producer's role on the Beatles' Let It Be album. Recorded at London's EMI Studios (now Abbey Road Studios), "Instant Karma!" employs Spector's signature Wall of Sound technique and features contributions from George Harrison, Klaus Voormann, Alan White, and Billy Preston. The B-side was "Who Has Seen the Wind?", a song composed and performed by Ono. When released in the US, the single was given a minor remix by Spector.

Recently shorn of the long hair synonymous with their 1969 campaign for world peace, Lennon and Ono promoted the single with an appearance on Britain's Top of the Pops five days after its release. The song received positive reviews and is considered by some music critics to be among the finest recordings from Lennon's solo career. A live performance recorded at his and Ono's "One to One" concerts in August 1972 was included on the posthumously released Live in New York City (1986). Paul Weller, Duran Duran, and U2 are among the acts who have covered "Instant Karma!" Its chorus also inspired the title to Stephen King's 1977 novel The Shining.

Everybody was going on about karma ... but it occurred to me that karma is instant, as well as it influences your past life or your future life. There really is a reaction to what you do now ... Also, I'm fascinated by commercials and promotion as an art form ... So, the idea of instant karma was like the idea of instant coffee: presenting something in a new form.

Together with his wife, Yoko Ono, John Lennon spent New Year 1970 in Aalborg, Denmark, establishing a relationship with Ono's former husband, artist Tony Cox, and visiting Cox's and Ono's daughter Kyoko. The visit coincided with the start of what Lennon termed "Year 1 AP (After Peace)", following his and Ono's heavily publicised Bed-Ins and other peace-campaign activities throughout 1969. To mark the new era, on 20 January 1970, the couple shaved off their shoulder-length hair – an act that Britain's Daily Mirror described as "the most sensational scalpings since the Red Indians went out of business." Having been recognised for his peace efforts in a segment on ITV's Man of the Decade documentary, and then chosen as Rolling Stone magazine's "Man of the Year" for 1969, Lennon said he cut his hair to "stop being hyped by revolutionary image and long hair." Lennon and Ono promised to auction the shorn hair for a charitable cause. This pledge followed the couple's announcement, on 5 January, that they would donate all future royalties from their recordings to the peace movement.

While in Denmark, the Lennons, Cox, and Cox's current partner, Melinde Kendall, discussed the concept of "instant karma", whereby the causality of one's actions is immediate rather than borne out over a lifetime. Author Philip Norman writes of the concept's appeal: "The idea was quintessential Lennon – the age-old Buddhist law of cause and effect turned into something as modern and synthetic as instant coffee and, simultaneously, into a bogey under the stairs that can get you if you don't watch out." On 27 January, two days after returning to the UK, Lennon woke up with the beginnings of a song inspired by his conversations with Cox and Kendall. Working at home on a piano, he developed the idea and came up with a melody for the composition, which he titled "Instant Karma!"

Lennon completed the writing of "Instant Karma!" in an hour. Eager to record the song immediately, he then telephoned his Beatles bandmate George Harrison and American producer Phil Spector, who was in London at the invitation of Allen Klein, the manager of the Beatles' Apple Corps organisation. According to Lennon's recollection, he told Spector: "Come over to Apple quick, I've just written a monster."

The song employs a descending three-note melodic progression similar to "Three Blind Mice" and an intro reminiscent of "Some Other Guy". Lennon had used a similar-sounding chord progression in the Beatles' 1967 single "All You Need Is Love". Later in 1970, he would adopt the melody of "Three Blind Mice", an English nursery rhyme, for his song "My Mummy's Dead".