On October 19, 2018, a lyric video of the song was released. The video is composed of computer-animated footage from the film. The lyrics are presented in the comic book style of the film, edited into the context of various scenes, such as graffiti scrawled by Miles Morales. Since its release, as of 2024, the lyric video has received over 2.5 billion views on YouTube.

On January 10, 2019, an official music video of the song was released, which is live action. The video contains an exclusive look at the recording session for the song showing the film logo behind Malone and Lee and a live performance. By 2023, the video had over 78 million views on YouTube.

The song received acclaim from music critics. In Billboard, Gil Kaufman called it "a funky, dreamy ballad" after listening to a preview of the song. Israel Daramola from Spin called "Sunflower" a "glitchy, soulful record" that "will appeal to fans of the more melodic songs made by Post and Swae Lee, as they commit to full crooning throughout. It’s a short and sweet, hazy little pop song that makes sense for a kid’s film or possibly an MTV show about California teens". Patrick Doyle of Rolling Stone characterized the "hyper-catchy" song as "a diary of a rocky relationship, which walks the line between hip-hop and dream-pop".

"Sunflower" debuted at number nine on the US Billboard Hot 100. After spending three weeks in the top five, it reached number one on the chart issue dated January 19, 2019, rising from number three the previous week. It became Post Malone's third and Swae Lee's first song as a soloist to top the chart. "Sunflower" is the first song from a soundtrack to reach the top of the Hot 100 since Justin Timberlake's "Can't Stop the Feeling!" from the Trolls soundtrack (2016). Additionally, "Sunflower" is the highest-charting song from a Spider-Man franchise soundtrack, passing Chad Kroeger's "Hero", featuring Josey Scott, from Music from and Inspired by Spider-Man, which reached number three in 2002. "Sunflower" also marks the first Hot 100 number one by two co-billed male leads with no other accompanying acts since George Michael and Elton John's "Don't Let the Sun Go Down on Me" (1992). Due to the large airplay lead of "Without Me" by Halsey, "Sunflower" descended to number two the following week. It continuously shuffled within the top five the following four months, going so far as to return to the runner-up spot in March and April. The song spent 33 non-consecutive weeks in the top ten of the Hot 100, tying both Ed Sheeran's "Shape of You" and Maroon 5's "Girls Like You", featuring Cardi B, for the longest top 10 run in the chart's archives. Malone's "Circles" broke the mentioned record in 2020, spending 38 consecutive weeks in the top 10. "Sunflower" topped the Canadian Hot 100 for two consecutive weeks.

According to the IFPI, "Sunflower" was the fourth most successful song of 2019, selling 13.4 million global equivalents-units. In the US, according to Nielsen Soundscan, it was the second-best performing R&B/hip-hop song of 2019, with 6.3 million equivalent units, including 668,000 digital downloads, and the fourth-best performing song on radio on that format.