The project first entered development in 2009. In March 2009, Variety reported that Warner Bros. would bring Stephen King’s 1986 novel to the big screen as a single feature film, with David Kajganich as screenwriter, and Dan Lin, Roy Lee and Doug Davison as producers. In June 2012, The Hollywood Reporter revealed that Cary Fukunaga was brought on to direct and co-write the script with Chase Palmer. In addition, Seth Grahame-Smith and David Katzenberg were added to the producing roster. In May 2014, it was announced that Warner Bros. had moved the film to its New Line Cinema division. In December, in an interview with Vulture, Dan Lin announced that the film would be split into two films, with the first film would be a coming-of-age story about the children tormented by It. Whilst the second will skip ahead in time as those same characters band together to continue the fight as adults. Also, Lin mentioned that the production was set for principle photography in the Summer of 2016. In March 2015, Fukunaga noted that his goal was to find the "perfect guy to play Pennywise", and, in May, it was officially announced that Will Poulter had been cast to play Pennywise, after Ben Mendelsohn, Ike Barinholtz, Richard Armitage, and Mark Rylance had all turned down the role. In the same month, Ty Simpkins was considered to play Bill Denbrough. Afterwards, it was reported that Fukunaga had suddenly dropped out of the project over creative differences. After Fukunaga's departure, King wrote, "The remake of IT may be dead—or undead—but we'll always have Tim Curry. He's still floating down in the sewers of Derry."
In July, it was announced that Andy Muschietti was in negociations to replace Fukunaga as director. Muschietti’s sister and collaborator, Barbara, was also brought on as producer. In October, Muschietti noted that the production would start shooting next Summer during an interview with Variety. In February 2016, Roy Lee confirmed that Fukunaga and Chase Palmer's original script had been rewritten. In April, it was indicated that Poulter had dropped out of the project due to scheduling conflicts. In addition, New Line Cinema were meeting with actors to replace Poulter, and had set the film for a September 8th 2017 release date. In June, both Bill Skarsgård and Jaeden Lieberher were both confirmed to portray Pennywise and Bill Denbrough, with a cast that included Finn Wolfhard, Jack Dylan Grazer, Wyatt Oleff, Chosen Jacobs, Jeremy Ray Taylor, and Sophia Lillis as the Loser Club. By the end of June, the rest of the cast was rounded out. In July, during an interview with Northumberland News, Barbara Muschietti revealed that filming locations for the film would take place throughout Toronto, Ontario, Canada. Principal Photography on the film began in Ontario, locations included Toronto, Port Hope, Oshawa, and Riverdale. Principal Photography wrapped in September, and Post-Production began in the same month. In July, the first offical image of Pennywise debuted. In August, the first full costume image of Pennywise was released in Entertainment Weekly. In February 2017, during the press tour for The Lego Batman Movie, Lin confirmed that It would be rated R. In March, New Line released the official teaser poster, and then a 139-second teaser trailer the following day. The trailer reached 197 million views in its first 24 hours, setting a new record as the trailer with the most views in one day. Beating out The Fate of the Furious (2017).