It (titled onscreen as It Chapter One) is a 2017 American supernatural horror film directed by Andy Muschietti and written by Chase Palmer, Cary Fukunaga, and Gary Dauberman. It is the first of a two-part adaptation of the 1986 novel of the same name by Stephen King, primarily covering the first chronological half of the book. It is the first film in the It film series as well as being the second adaptation following Tommy Lee Wallace's 1990 miniseries. Starring Jaeden Lieberher and Bill Skarsgård, the film was produced by New Line Cinema, KatzSmith Productions, Lin Pictures, and Vertigo Entertainment. The film, set in Derry, Maine, tells the story of The Losers' Club (Lieberher, Sophia Lillis, Jack Dylan Grazer, Finn Wolfhard, Wyatt Oleff, Chosen Jacobs, and Jeremy Ray Taylor), a group of seven outcast children who are terrorized by the eponymous being which emerges from the sewer and appears in the form of Pennywise the Dancing Clown (Skarsgård), only to face their own personal demons in the process.

Development of the theatrical film adaptation of It began in March 2009 when Warner Bros. started discussing that they would be bringing it to the big screen, with David Kajganich planned to direct, before being replaced by Fukunaga in June 2012. After Fukunaga dropped out as the director in May 2015, Muschietti was signed on to direct the film in June 2015. He talks of drawing inspiration from 1980s films such as The Howling (1981), The Thing (1982) The Goonies (1985), Stand by Me (1986) and Near Dark (1987) and cited the influence of Steven Spielberg. During the development, the film was moved to New Line Cinema division in May 2014. Principal photography began in Toronto on June 27, 2016, and ended on September 21, 2016. The locations for It were in the Greater Toronto Area, including Port Hope, Oshawa, and Riverdale. Benjamin Wallfisch was hired in March 2017 to composed the film's musical score.

It premiered in Los Angeles at the TCL Chinese Theatre on September 5, 2017, and was released in the United States on September 8, 2017, in 2D and IMAX formats. A critical and commercial success, the film set numerous box office records and grossed over $704 million worldwide, becoming the third-highest-grossing R-rated film at the time of its release. Unadjusted for inflation, it became the highest-grossing horror film of all time. It received generally positive reviews, with critics praising the performances, direction, cinematography and musical score, and many calling it one of the best Stephen King adaptations. It also received numerous awards and nominations, earning two Washington D.C. Area Film Critics Association nominations, including Best Acting Ensemble. It was nominated for the Critics' Choice Movie Award for Best Sci-Fi/Horror Movie. The film won three Bogey Awards, for pulling in more than two million German admissions in 11 days. In addition, the motion picture was named one of the best films of 2017 by various critics, appearing on several critics' end-of-year lists. The second film, It Chapter Two, was released on September 6, 2019, covering the remaining story from the book.