It was released by Interscope Records, Sony and Ultra on 16 February 2017 as the lead single from Kygo's debut extended play Stargazing (2017) and appears as an international bonus track on Gomez's third studio album, Rare (2020).

An EDM, dance-pop, electropop and tropical house song, "It Ain't Me" comprises an acoustic guitar line, and a build-drop arrangement in its chorus featuring pulsing piano notes, bass, synthesizers, finger-snap claps and pan flute melodies.

In addition to reaching number one in Croatia, Lebanon, and Norway, the song attained top five peaks in Australia, Austria, Belgium, Canada, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Ireland, Malaysia, the Netherlands, New Zealand, Poland, Portugal, Scotland, Slovakia, Sweden and Switzerland.

Tamposi recalled, "When they pushed the space bar to play the song, [Kygo] went from this very sweet, polite, charming young man to this super-human producer guy.

[3][4] Tamposi was called back for the vocal sessions with Gomez where they shared a similar emotional vision for "It Ain't Me", basing it on their own personal experiences.

[8] According to the official sheet music at Musicnotes.com, "It Ain't Me" is composed in the key of A minor and is set in the time signature of alla breve at a tempo of 100 beats per minute.

The song carries a calm tone and a drop common in tropical house music, but is complemented by an equal measure of electronic and pop rock elements.

[10] The production is subtle and varied with finger snaps sometimes being louder than the kick drum, and the guitar riff and piano notes alternating in and out.

In the drop Kygo introduces sidechained vocal chops of Gomez singing syllables and are accompanied by percussion instruments.

[19] Tamposi explained that "It Ain't Me" is about a woman finding the strength to walk away from a toxic relationship despite the pressure she receives from society to stand by her man.

[20] In early February 2017, "It Ain't Me" was registered on the American Society of Composers, Authors and Publishers (ASCAP) database.

[22] Gomez later revealed the single's cover art and release date on Instagram on 13 February 2017, and shared three teasers of the song on Snapchat the following day.

[23][24] "It Ain't Me" was released for digital download by Interscope, Sony and Ultra Music on 17 February 2017 as the first single from Kygo's upcoming second studio album.

[27] Matt Medved of Billboard regarded it as one of Gomez's most mature-sounding releases and appreciated Kygo's usage of her vocals, adding that the song showed both collaborators' "best sides".

[15] In a negative review, Pitchfork described it as "yet another exercise in EDM guys reducing female vocalists to anonymities," deeming it inferior to Gomez's follow-up single "Bad Liar".