IO Interactive A/S (IOI) is a Danish video game developer and publisher based in Copenhagen, best known for creating and developing the Hitman and Kane and Lynch franchises. IO Interactive's most recent game is Hitman 3, which was released in January 2021. Their next game will be 007 First Light, to be released in 2026.

The company was founded in September 1998 as a joint venture between the seven-man development team of Reto-Moto and film studio Nordisk Film. IO Interactive was acquired by publisher Eidos Interactive for £23 million in March 2004, which saw itself acquired by Square Enix and renamed as Square Enix Europe in 2009. In May 2017, Square Enix ceased funding for IO Interactive and started seeking a buyer for the studio. IO Interactive performed a management buyout in June 2017, becoming independent and regaining the rights to their Hitman and Freedom Fighters franchises. IO Interactive employs over 500 people as of June 2025 and operates four subsidiary studios: IOI Malmö in Malmö, Sweden; IOI Barcelona in Barcelona, Spain, IOI Istanbul in Istanbul, Turkey, and IOI Brighton in Brighton, England.

In 1997, Reto-Moto was founded as a video game developer in Copenhagen. Before the studio finished any games, they struck a partnership with Danish film studio Nordisk Film in 1998 that would lead to the creation of a developer jointly owned by the two companies. The resulting company, IO Interactive, was established on 16 September 1998, with Reto-Moto's seven employees—Jesper Vorsholt Jørgensen, Rasmus Guldberg-Kjær, Martin Munk Pollas, Karsten Lemann Hvidberg, Jacob Andersen, Janos Flösser, and David Guldbrandsen—serving as the founders and initial staff of IO Interactive. Nordisk Film and the seven founders each held a 50% stake in the venture. By March 2004, Nordisk Film owned 40.3% in IO Interactive.

Early on, IO Interactive conceptualised a fantasy massively multiplayer online game (MMO) entitled Rex Dominus, however, Nordisk Film staff asked the development team to cease production on Rex Dominus and "test [themselves]" by developing a "simple shooter" instead. As such, the team opted for a run-and-gun action game, as it took less time to develop compared to an MMO, drawing inspiration from John Woo films, such as Hard Boiled and The Killer. They turned to develop for personal computers (PCs), because they were unable to acquire development kits for consoles, and had also found interest in the steady increase of PCs' 3D graphics capabilities.

A part of the development on the game, which would later become Hitman: Codename 47, was the creation of the Glacier, the studio's proprietary game engine that fit their needs; co-founder Andersen stated: "Since killing was the main theme of the game, we wanted to do something special. [...] Standard 'death animations' just looked too static so some of the coders tried to see if they could use real-time inverse kinematics for the falling bodies. The first versions ran terribly slowly until one of the programmers figured out a way to fake the whole calculation." This led to the first use of advanced ragdoll physics in a video game. This physics system caught the eyes of British publisher Eidos Interactive, and especially staff member Jonas Eneroth, who thought that the system could greatly benefit Codename 47's gameplay. Following six months of negotiations, a publishing deal was signed between IO Interactive and Eidos Interactive. Eneroth became executive producer on the project.

As executive producer, Eneroth encouraged the development team to stray away from the run-and-gun gameplay, and instead focus on a "methodical experience", including dragging dead bodies around the scene to create tension. He had previously worked on Deus Ex and Thief: The Dark Project, which had heavily exposed him to the stealth game mechanics he wished to see in Codename 47. The game was released on 19 November 2000, with reception mixed due to the difficulty of the game.

In October 2001, Eidos Interactive announced Hitman 2: Silent Assassin, a sequel to Codename 47. Following a slight delay in March 2002, it was released in October 2002, this time for Microsoft Windows, as well as PlayStation 2 and Xbox. To more easily overcome the challenge of bringing the game to consoles, IO Interactive grew "considerably" in headcount. The game was received well by critics; according to Greg Kasavin in his review for GameSpot, "Hitman 2 fixes virtually all of the problems of its predecessor". The success of Silent Assassin came as a surprise to the team, and was swiftly followed up with by porting the game to GameCube the following year.

In 2003, IO Interactive decided to open a Hungarian offshoot, named IO Interactive Hungary. To properly establish the studio, 50 Hungarian staff were hired and brought to the company's Copenhagen headquarters for a six-month training programme. However, after the training had been finished, IO Interactive realised that there were 50 people with talent but no leadership that could guide them when in Hungary. Instead of fulfilling the Hungarian subsidiary, IO Interactive opted to offer all 50 people jobs at their headquarters, to which most of them agreed. In October that year, Electronic Arts released Freedom Fighters, an IO Interactive-developed third-person shooter which was previously announced as Freedom: The Battle for Liberty Island in May 2002. Although a sequel to Freedom Fighters has been anticipated, IO Interactive has been unable to comment on whether such a game was in development.