In its third week the film was dethroned by newcomer Kingsman: The Golden Circle, finishing second at the box office with $29.7 million. In its fourth week, the film initially made a projected gross of $17.3 million, apparently retaking its top spot at the box office ahead of Kingsman ($17 million). However the following day, actual results had the film finishing in second by a gross of $16.93 million to $16.90 million, while beating out newcomer American Made ($16.8 million). The film continued to hold well in the following weeks, making $10 million and $6.1 million in its fifth and sixth weeks, finishing a respective 3rd and 4th at the box office.

It went on to open to $123.1 million, setting the records for largest opening weekend for both a September release and a horror film, and was the second-biggest debut for an R-rated film behind Deadpool ($132.4 million). Variety and Deadline both noted that the film's opening weekend could have been even greater if not for Hurricane Irma shutting down nearly 50% of Florida's theaters, a state that typically accounts for 5% of the country's box office grosses. During its first full week, the film made $8.8 million on Monday, $11.4 million on Tuesday, $7.9 million on Wednesday and $7.2 million on Thursday, each setting September records for their respective days. In its second weekend the film grossed $60.1 million (a better-than-average for horror films drop of 51%), making more in its second weekend than previous opening record holder Paranormal Activity 3 made in its first, and again topped the box office. It also pushed the domestic total to $218.7 million, overtaking Crocodile Dundee for highest-grossing September film ($174.8 million in 1984).

In October 1988, Bill Denbrough gives his seven-year-old brother, Georgie, a paper sailboat. Georgie sails the boat along the rainy streets of small town Derry, and is disappointed when it falls down a storm drain. As he attempts to retrieve it, Georgie sees a clown in the sewer, who introduces himself as Pennywise the Dancing Clown. The clown entices Georgie to come closer, then severs his arm and drags him into the sewer.

On March 12, 2009, Variety reported that Warner Bros. would bring Stephen King's novel to the big screen, with David Kajganich adapting the novel, and Dan Lin, Roy Lee and Doug Davison producing the piece. When Kajganich learnt of Warner Bros.' plans to adapt King's novel, he went after the job. Knowing that Warner Bros. was committed to adapting It as a single feature film, Kajganich began to reread the novel in an attempt to try to find a structure that would accommodate such a large number of characters in two different time periods, around 120 pages, which was one of Warner Bros.' stipulations. Kajganich worked with Lin, Lee, and Davison on The Invasion (2007), and he knew they would champion good storytelling, and allow him the time to work out a solid first draft of the screenplay. Kajganich spoke of the remake being set in the, "mid-1980s and in the present ... mirroring the twenty-odd-year gap King uses in the book ... and with a great deal of care and attention paid to the backstories of all the characters".

Kajganich also mentioned that Warner Bros. wished for the adaptation to be rated R, saying, "... we can really honor the book and engage with the traumas (both the paranormal ones and those they deal with at home and school) that these characters endure". He said that his dream choice for Pennywise would be Buster Keaton if he were still alive, and that the Pennywise that he scripted is "less self-conscious of his own irony and surreality". As of June 29, 2010, Kajganich was re-writing his screenplay.

On June 7, 2012, The Hollywood Reporter said that Cary Fukunaga was boarding the project as Director, and would co-write the script with Chase Palmer. The producers were now Roy Lee of Vertigo Entertainment, Dan Lin of Lin Pictures, and Seth Grahame-Smith and David Katzenberg of KatzSmith Productions. On May 21, 2014, Warner Bros. was announced to have moved the film to its New Line Cinema division, with overseer duties conducting by New Line's Walter Hamada and Dave Neustadter, along with Vice President of Production at Warner Bros., Niija Kuykendall. On December 5, 2014, in an interview with Vulture, Dan Lin announced that the first film would be a coming-of-age story about the children tormented by It, and the second will skip ahead in time as those same characters band together to continue the fight as adults. Lin also stated that Fukunaga was then only committed to directing the first film, though was currently closing a deal to co-write the second. Lin concluded by mentioning King, to which he remarked, "The most important thing is that [King] gave us his blessing. We didn't want to make this unless he felt it was the right way to go, and when we sent him the script, the response that Cary got back was, 'Go with God, please! This is the version the studio should make.' So that was really gratifying." Lin confirmed that Fukunaga was set to begin principal photography in the summer of 2016.

In North America, initial opening weekend projections had the film grossing $50–60 million. By the week of its release, estimates were raised to $60–70 million, with a chance to go higher if word of mouth was strong. It opened in 4,103 theaters, setting the record for most venues for an R-rated film (beating Logan's 4,071 from the past March). A few days before its release, the film became Fandango's top horror pre-seller of all-time, eclipsing Paranormal Activity 3 (2011), as well as setting the record as the site's top pre-seller among September releases, beating Sully (2016). The film made $13.5 million from Thursday night previews, setting the record for highest amount by both an R-rated (besting Deadpool's $12.6 million) and a horror film. Due to the high Thursday gross, Deadline.com noted some industry trackers upped weekend projections to $90 million. It went on to have an opening day of $50.2 million (including previews), increasing weekend projections to over $100 million. The film's Friday gross not only set a record for biggest single-day amount by an R-rated film (beating Deadpool's $47.3 million) but nearly eclipsed Paranormal Activity 3's entire weekend gross of $52.6 million, which was the highest opening weekend gross for a horror film.

The following summer, Bill and his friends—Richie Tozier, Eddie Kaspbrak, and Stan Uris—run afoul of older bully Henry Bowers and his gang. Bill, still haunted by Georgie's disappearance and the resulting neglect from his grief-stricken parents, discovers that his brother's body may have washed up in a marshy wasteland called the Barrens. He recruits his friends to investigate, believing his brother may still be alive. Ben Hanscom learns that the town has been plagued by unexplained tragedies and child disappearances for centuries. He is targeted by Bowers' gang, after which he flees into the Barrens and meets Bill's group. They find the sneaker of a missing girl, while a member of the pursuing Bowers Gang, Patrick Hockstetter, is killed by Pennywise while searching the sewers for Ben.

On October 30, 2015, Muschietti was interviewed by Variety wherein he spoke about his vision of It, while mentioning Poulter was still in the mix for the role of Pennywise: "[Poulter] would be a great option. For me he is at the top of my list ...." He confirmed that next summer is the time for them to start shooting. It was decided to shoot It during the summer months to give the filmmakers time to work with the children who have the main roles in the first part of the film. Muschietti went on to say that "King described 50s' terror iconography", adding that he feels there is a whole world now to "rediscover, to update". He said there would not be any mummies or werewolves and that the "terrors are going to be a lot more surprising". On February 19, 2016, at the D.I.C.E. Summit 2016, Producer Roy Lee confirmed that Fukunaga and Chase Palmer's original script had been rewritten, remarking, "It will hopefully be shooting later this year. We just got the California tax credit ... [Dauberman] wrote the most recent draft working with [Muscetti], so it's being envisioned as two movies."

On February 16, 2016, Producer Roy Lee, in an interview with Collider.com, mentioned a second film, remarking that: "[Dauberman] wrote the most recent draft working with [Muschietti], so it's being envisioned as two movies." On July 19, 2017, Muschietti revealed that the plan is to get production underway for the sequel to It next spring, adding, "We'll probably have a script for the second part in January [2018]. Ideally, we would start prep in March. Part one is only about the kids. Part two is about these characters 30 years later as adults, with flashbacks to 1989 when they were kids." On July 21, 2017, Muschietti spoke of looking forward to having a dialogue in the second film that does not exist within the first, stating, "... it seems like we're going to do it. It's the second half, it's not a sequel. It's the second half and it's very connected to the first one." Muschietti confirmed that two cut scenes from the film will hopefully be included in the second, one of which being the fire at the Black Spot from the book.

On September 25, 2017, New Line Cinema announced that the sequel would be released on September 6, 2019, with Gary Dauberman writing the script. Later in December 2017, Agent Cody Banks Writer Jeffrey Jurgensen was also listed as a Screenwriter. As of February 2018, Jessica Chastain was in negotiations to star as the adult Beverly Marsh. In April 2018, James McAvoy and Bill Hader entered negotiations to play the adult Bill Denbrough and Richie Tozier.

A sequel, It: Chapter Two, is scheduled to be released on September 6, 2019.