The film was officially announced in July 2016, although Legendary Pictures were reported to be in talks as early as April of that year. The Pokémon Company and Letterman wanted to adapt Detective Pikachu because of their interest in making a film that focused on another character besides Ash Ketchum, the protagonist of the Pokémon animated TV series. On the premise, Letterman stated, «The Pokémon Company, they’ve already made many, many movies of Ash, and they came to Legendary with this idea of using a new character. So when I came onboard, I was pitched this character of Detective Pikachu, and I fell in love with the story behind it.» The idea of talking Pokémon originated from an early concept for the 1990s TV series, but was scrapped when the original game developer, Game Freak, was unsatisfied with the concept. The idea was revived for the 2016 Detective Pikachu spin-off game. Letterman said that they «spent a year designing all the characters ahead of shooting so that we could get it all right.»
Rob Letterman was hired to direct the film on November 30, 2016, and the studio fast-tracked production to start in 2017. On August 16, 2016, Nicole Perlman and Alex Hirsch were in negotiations with Legendary to write the screenplay. Later revisions were provided by Eric Pearson, Thomas McCarthy, Derek Connolly, Dan Hernandez, Benji Samit and Letterman. Ultimately, Hernandez, Samit, Letterman, and Connolly received screenplay credit, and with Hernandez, Samit and Perlman receiving «story by» billing.
In November 2017, Justice Smith was cast in the lead human role, with Kathryn Newton added to costar after an intense session of reading and testing actresses opposite Smith. Newton beat out Natalia Dyer, Haley Lu Richardson, and Katherine Langford for the role. In December 2017, Ryan Reynolds was cast in the title role, portrayed via motion-capture and voice over. Other actors considered for the role were Danny DeVito, Dwayne Johnson, Mark Wahlberg and Hugh Jackman. In January 2018, with production commencing, Ken Watanabe, Bill Nighy and Chris Geere joined the cast, in February 2018, Suki Waterhouse and Rita Ora were added as well, and in April 2018, Omar Chaparro had been signed. In January 2019, Rob Delaney had previously stated that he had a role, but he does not appear in the final cut of the film.
Principal production began on January 15, 2018, in London, England and Denver, Colorado. Nine days later, Legendary announced that principal photography had officially begun. Much of the on set interaction and vocal reference for Pikachu was filled in by Jon Bailey. However, all of his dialogue was dubbed over by Ryan Reynolds. Principal photography concluded on May 1, 2018. Some filming was done at Shepperton Studios, Warner Bros. Studios, Leavesden, and Minley Woods in Hampshire; rural areas of Colorado, just outside Denver and Colorado Springs; and Scotland.
The film’s cinematographer, John Mathieson, noted that, like his other films, Detective Pikachu was shot on traditional film, in contrast to most other contemporary films which are shot digitally. He said the use of traditional film helps make it «look more realistic.»