In addition to writing the story, Rakesh Roshan also wrote the screenplay with Sachin Bhowmick, Honey Irani, and Robin Bhatt.

Koi... Mil Gaya focuses on Rohit Mehra, a developmentally disabled man who contacts an extraterrestrial being later named Jadoo with his late father Sanjay's supercomputer.

Principal photography was shot by Ravi K. Chandran and Sameer Arya from November 2001 to March 2003 on sets built by Sharmishta Roy in Canada, India, and New Zealand.

[24] Koi... Mil Gaya was produced by Rakesh Roshan under Filmkraft Productions, which he established in 1980, and distributed by Yash Raj Films.

[35] To provide an accurate portrayal, Hrithik Roshan lost 8 kilograms (18 lb), changed his hairstyle, and wore loose clothes to cover his well-built body.

[18][31] Zinta was given the role of Rohit's friend and then-wife, Nisha, after Rakesh Roshan saw her 20-minute performance in Mani Ratnam's 1998 thriller Dil Se.. and was impressed by it.

[43] In July 2001, Rekha joined the cast and portrays Rohit's mother Sonia, a part that was specifically written by Rakesh Roshan for her.

), three foot (nearly one metre) animatronical mask, which was created by the Australian artists James Colmer and Lara Denman from Bimini Special Effects Studios.

[9] With help from the Bangalore-based company Compudyne Winfosys, the American artists Marc Klobe and Craig Mumma, who previously collaborated on the science fiction films Independence Day (1996) and Godzilla (1998), were involved in the special effects.

[2] Nonetheless, the director confessed that the film's total cost was still excessive for him and attributed this problem to the special effects and making Jadoo's costumes.

"[64] Amod Mehra said that the film's themes would successfully attract an audience, giving "adults ... a chance to relive their childhood".

[64] Sharing the same sentiments, the critic and film trade observer Taran Adarsh added that "the combination of Hrithik, Rakesh and Rajesh Roshan" would make its opening "bumper".

[75] Koi... Mil Gaya garnered high expectations from trade analysts as well, which left Hrithik Roshan restless.

[85][86] Koi... Mil Gaya was successful at the box office, due to the audience's sympathy for Hrithik Roshan's character as a man with developmental disability.

[87][88] The film was released on 450 screens across India and grossed ₹22.5 million (US$260,000) on its opening day, the year's second-highest first-day earnings.

[87][99] Rekha and Zinta's performances were panned; several critics found them to be "utterly wasted" serving only as the film's "decorative piece[s]".

[101] Udita Jhunjhunwala of Mid-Day observed, "You walk into Koi... Mil Gaya expecting to find a mysterious alien, what you end up discovering is a child inside you."

[105] Writing for The Afternoon Despatch & Courier, Deepa Gahlot described Koi... Mil Gaya as "a great demo of the Bollywood style of filmmaking", but criticised its scientific inaccuracy.

[107] Screen's editor Bhawana Somaaya appreciated Hrithik Roshan's performance, finding it to be "very sincere" and "brave"; she also took note of the film's "refreshing" subject.

[115] In his five-star review published by B4U, Parag Chandrabala Maniar lauded the efforts of Rakesh Roshan "to provide pure and clean entertainment" and Kolbe's and Mumma's special effects.

[117] The Times of India's Omar Qureshi elaborated, "Hrithik blows away all doubts, theories, criticisms and disbelief with an extraordinarily touching performance of a mentally challenged boy in a rough man's world.

Ed Halter of The Village Voice predicted that the film's elements, including its "nonstop plagiarism from classics like E.T., Star Wars, and Close Encounters of the Third Kind", might be embraced by foreign nerds.

[122] Calling it "bright, loud and relentless fun", Jürgen Fauth gave the film three-and-a-half-out-of-five stars and reviewed its comedic, romantic, science-fictional, musical, action, and melodramatic elements positively.

[125] Derek Elley praised him for avoiding his typecasting as a romantic hero,[126] while David Parkinson of Radio Times thought that the film served as "slick, sentimental entertainment".

[138]:56 Writing for The New York Times, the critic and author Anupama Chopra stated that Koi... Mil Gaya was "Hindi cinema's first major science fiction film.

"[139] The BBC reported that the film "is certainly a milestone in the reformation of commercial Indian cinema as a whole",[140] and the American graphic artist Aaron Marcus, in his 2014 book Design, User Experience, and Usability: Health, Learning, Playing, Cultural, and Cross-Cultural User Experience called it the most popular Bollywood science fiction film.

[17][145] The release of Koi... Mil Gaya, which Hrithik Roshan wanted to revive his career, changed analysts' perspectives of the actor and suggested that the film resurrected his position in the industry.

"[72] In 2010, Filmfare magazine included his performance on their list of "80 Iconic Performances" of Hindi cinema, writing, "It's incredible how the actor manages to diminish his size, unstylise his look, wear bug-eyed glasses and talk funny ... Watch him laugh, cry or bond with his remote controlled alien friend and note his nuanced turn.

Nickelodeon India aired a spin-off television series based on the character, titled J Bole Toh Jadoo, in 2004.

[153] In 2020, after the United States Department of Defense published footage of an unidentified flying object, internet memes relating to the film went viral; most of them use the scene where Rohit summons aliens by his father's computer.