"Bellyache" was released for digital download and streaming on February 24, 2017, as the second single from Eilish's debut EP Don't Smile at Me, through Darkroom and Interscope Records.

[2] In December 2021, American rapper Doja Cat revealed that she was asked by Eilish to feature on the track during its early stages of development, yet declined since she was experiencing writer's block at the time.

"[4] A remix of the song was created by Marian Hill, an American songwriting duo, and was released on May 5, 2017, for digital download.

[13] Jason Lipshutz of Billboard mentioned that "Bellyache" has "macabre lyrics around a delectable pre-chorus hook and a steady acoustic pattern that eventually gives way to a sizzling drum machine".

[11] The song's lyrics were written from an entirely fictional perspective of a psychopath who kills those she is close to, including her friends and lover.

[14] Eilish takes on the persona of the psychopath who has killed her friends and dumped their bodies in the back seat of her car.

[11] Meaghan Garvey, writing for The Fader, cited the track as a "loping" and "cowboy-ish number with a touch of deep-house bounce".

[8] Mark Savage lauded the song in his review for BBC News, saying it is the "pop equivalent" to the works of Quentin Tarantino, and finds "comic absurdity in the midst of eye-popping gore".

She elaborated, mentioning that "the casual guitar strums coupled with Eilish unapologetic embodiment of [the] character and beats really challenge you not to move to them".

[15] Insider's described the lyrics as "violent" and "dark", commenting that they "belie the song's quietly upbeat melody", and prove Eilish and Finneas as "some of the most talented and versatile artists making music today".

[26] It was also awarded a 5× platinum certification by the Australian Recording Industry Association (ARIA) for selling 350,000 track-equivalent units in the country.

[30][31] An accompanying music video was uploaded to Eilish's YouTube channel on March 22, 2017, and was directed by Miles and AJ.

[18] The video begins with Eilish walking down a long, empty desert road, dragging a red wagon filled with garbage bags, which she eventually reveals to be full of cash.

De Elizbeth of Teen Vogue said the video "definitely invokes feelings of summertime", and that it will "make you want to find your best romper and sunnies and be outside".

[33] Writing for Nylon magazine, Sydney Gore compared Eilish's look in the visual to that of Uma Thurman in the 2003 American film Kill Bill.

[32] Gore also praised the choreography, saying that "we're experiencing hallucinations as we watch her bust out some sick down the road which leads to the great unknown".

[34] Nate D. Sanders auctioned a yellow rain suit and jacket nearly identical to the outfit worn by Eilish in the music video for 12,000 dollars in February 2020.

[43] In December 2019, Eilish performed "Bellyache" at the Steve Jobs Theater for the first annual Apple Music Awards after she won artist of the year, with O'Connell playing the piano.