In mid 2012, Ron Perlman once again endured the 4-hour makeup routine required to transform him into Hellboy, to fulfill the Make-A-Wish request of a six-year-old boy named Zachary who had leukemia. Creature effects house Spectral Motion, who had worked on the two previous Hellboy films, applied Perlman's Hellboy makeup (and later, also made up Zachary as Hellboy as well), so that Zachary could spend the day hanging out with his favorite superhero. Guillermo del Toro was so touched by this event that it inspired him to re-start production on the shelved Hellboy 3. However, after several years without progress because del Toro was working on several other projects, the studio had approached Neil Marshall to helm the next film, and decided that it had to be a reboot to set it apart from del Toro's vision. Del Toro's Hellboy III was subsequently announced to be canceled, and this reboot's development started soon after.
Mike Mignola takes a more direct involvement in the production of this film than he did on the Guillermo del Toro Hellboy films. According to him, since a new director would helm the reboot, the idea with this one was to downplay the superhero elements in the film, strip out the "del Toro-isms", and stay closer to the darker tone of the graphic novels, making it a much more horror-oriented entry into the franchise.
David Harbour described his portrayal of Hellboy is a teenage version of Ron Perlman's Hellboy: "He's younger and tougher, and really struggling with the idea of whether or not he's a good person."
A follow-up to Hellboy II: The Golden Army (2008) had long been in development hell, mostly due to a projected budget of $120 million whereas the first two movies had only seen modest returns, and because director Guillermo del Toro was unavailable for a long time. Following the success that Deadpool (2016) had in getting the movie made through social media, Ron Perlman tweeted a proposal that if #HellboyIII started trending, a third Hellboy film might finally get the funds to be made. Del Toro soon followed suit, but by the time that he was finally ready to discuss the project, the studio had already decided to make this completely unrelated reboot.
On August 21, 2017, Ed Skrein was cast as Major Ben Daimio in the film. However, upon discovering that Daimio was portrayed in the comic books as a Japanese-American character, Skrein announced a week later that he was pulling out to allow an actor of Asian heritage to be cast instead. According to The Hollywood Reporter, Skrein's decision marked the first time a white actor had left a high-profile role after such criticism.
Neil Marshall: [the main character covered in blood during an intense situation] There are some scenes in which Hellboy is covered in blood due to a violent or bizarre occurrence, as was Sarah Carter in The Descent (2005).