I just think it's ridiculous, so funny and stupid. We heard the strings that sounded so Disney and orchestral, and full and pure. And I was just like, "Yo, what's the dirtiest possible, most opposing lyric that we could write to this?"— Grande, during an interview on the Zach Sang Show.

"34+35" was first revealed when Grande announced the track list of Positions via social media on October 24, 2020. Grande started working on the song in 2019, and finished it in September 2020 after she decided it was going to be on the album. It was originally intended to be the lead single of the album before being replaced by title track "Positions", after Grande decided that it was more appropriate as it gave a more accurate representation of the album. Given the explicit nature of the song, which has been compared to Grande's past works such as "Side to Side" and "Dangerous Woman", Grande said that she was "very nervous" about the song initially, fearing that it would "distract from the vulnerability and the sweetness that is the rest of the album". She stated that it was sonically "one of [her] favorite things [she and her team have] ever done", adding: "it deserve[d] a home on the album for sure...I think that everything I do has a little bit of humor, and the people know that I'm not really sitting here 'til dawn."

To promote the song, Grande released a "34+35 dad hat" on her official merchandise store, which features "34+35" graphics embroidered on the front of a baseball cap. "34+35" was sent to contemporary hit radio in Australia and the United States on October 30 and November 3, 2020, respectively. It was later sent to hot adult contemporary radio on January 19, 2021. A lyric video was released on YouTube on October 30, 2020.

"34+35" is an uptempo pop song with influences of R&B and an instrumentation utilizing violins, plucked strings, and elements of trap music. The song is written with a tempo of 110 beats per minute in the key of F major, while Grande's vocals range from a low note of G3 to a high note of F5. The song's lyrics include sexual puns, double entendres, and sex jokes, while the song's title (which, as a mathematical expression, evaluates to 69) and chorus reference to the 69 sex position, with the outro's lyrics including "Means I wanna 69 witcha". Other lyrics include explicit sexual references, such as "Can you stay up all night?/Fuck me 'til the daylight". The song was written about Grande's sex life with her boyfriend Dalton Gomez. According to Grande, the line, "Just gimme them babies" was suggested by songwriter Scott Nicholson as a joke to complete the first verse, on the night they started writing the song, but she liked it so much that it ended up in the final version. The earthquake referenced in the third verse ("Got the neighbours yellin' "Earthquake!"/4.5 when I make the bed shake") was based on an actual earthquake of magnitude 4.5 that occurred in Southern California in September 2020. Grande wrote the bridge together with songwriters Tayla Parx and Victoria Monét.

Mary Siroky of Consequence of Sound called the track "overwhelmingly playful" and "no-holds-barred", with Heather Taylor-Singh of Exclaim! writing that it featured an "overt sexiness". Adam White of The Independent called the song's lyrics "dirty-minded" and "cutely tongue-in-cheek", while Brenton Blanchet of Clash regarded "34+35" as "Grande's most sexually explicit track yet". Similarly, Natalie Morin of Refinery29 named the song "Grande's sex magnum opus".

On his ranking of every song on Positions, where "34+35" was positioned at number two, Jason Lipshutz of Billboard praised Grande for being one of the "few artists working today [who] could release an explicit sex track that sounds this luxurious and ornately detailed", describing the song as "unapologetic and dizzyingly fun, with Grande shrugging off the need for euphemism and rightfully confident in her approach". Writing for The Line of Best Fit, Ross Horton identified "34+35" as the best song on and the "mission statement" for the album, writing that "the amount of puns, double entendres and jokes littered throughout is staggering".

Variety's Chris Willman remarked that "34+35" was "a little bit on the schoolgirl-sniggering side" and that the outro's literalization of the song's 69 joke was "unnecessary". Shaad D'Souza of The Fader referred to the song as a "hammy ode to 69-ing" which "makes no attempts at even the most vaguely clever innuendo". Slant Magazine's Alexa Camp considered the song's lyrics "a lot of empty pillow talk and plentiful dog whistles." Emma Madden of Entertainment Weekly criticized the song's lyrics, writing, "Grande sings that she was 'never good' in math class on '34+35,' and it seems she might be equally bad at math-related wordplay."
Uproxx ranked "34+35" as the 9th best song of 2020, with Caitlin White commenting that the song is more carefree and funnier than the lead single "Positions", and that it "has all the makings of a late album hit, and will go down in Ari's catalogue as one of her most clever offerings". LaTesha Harris from NPR ranked "34+35" as the 8th best song of 2020. Tidal ranked "34+35" as the 32nd best pop song of 2020. Jessica McKinney of Complex ranked "34+35" as the 42nd best song of 2020. Calling its production "immaculate", she commented that the song is "sexually explicit, seductive, luxurious, and elegant, guided by Grande's sultry vocals".