Mamoru Hosoda and Jin Kim both had an admiration for each other's work. While Hosoda was attending the Oscar's ceremony for his film, Mirai (2018) the two were able to meet for the first time. It was there the two said they would work together on a future project, which eventually became Belle.
The main theme and first song heard in the movie is by Japanese music group Millennium Parade, and there are many subtle hints and references to the group within the movie. Peggie Sue is voiced by Ermhoi, who frequently provides vocal work in their songs. One of the unnamed characters pursuing the Dragon has a dog face, whose appearance is almost identical to the dog characters in the music video of Millennium Parade's Philip. If you look closely you can also spot other characters that resemble people in the dance scene in their video for Bon Dance as well.
This film marks Studio Chizu's 10th anniversary.
Motion picture debut of Ludvig Forssell; Forssell is best known for composing music for Hideo Kojima's video games, most prominently the Metal Gear Solid series.
Hiro owns a copy of Pete the Cat, seen standing on a table in her computer room at her home.