Doom is a 1993 first-person shooter (FPS) game developed by id Software for MS-DOS. Players assume the role of a space marine, popularly known as Doomguy, fighting their way through hordes of invading demons from hell. Id began developing Doom after the release of their previous FPS, Wolfenstein 3D (1992). It emerged from a 3D game engine developed by John Carmack, who wanted to create a science fiction game inspired by Dungeons & Dragons and the films Evil Dead II and Aliens. The first episode, comprising nine levels, was distributed freely as shareware; the full game, with two further episodes, was sold via mail order. An updated version with an additional episode and more difficult levels, The Ultimate Doom, was released in 1995 and sold at retail.

Doom is one of the most significant games in video game history, frequently cited as one of the greatest games ever made. It sold an estimated 3.5 million copies by 1999; between 10 and 20 million people are estimated to have played it within two years of launch, and in late 1995, it was estimated to be installed on more computers worldwide than Microsoft's then-new operating system, Windows 95. Along with Wolfenstein 3D, Doom helped define the FPS genre and inspired numerous similar games, often called Doom clones. It pioneered online distribution and technologies including 3D graphics, networked multiplayer gaming, and support for custom modifications via packaged WAD files. Its graphic violence and supposed hellish imagery drew controversy from different groups, such as parents and the news.

Doom has been ported to numerous platforms. The Doom franchise continued with Doom II: Hell on Earth (1994) and expansion packs including Master Levels for Doom II (1995). The source code was released in 1997 under a proprietary license, and then later in 1999 under the GNU General Public License v2.0 or later. Doom 3, a horror game built with the id Tech 4 engine, was released in 2004, followed by a 2005 Doom film. id returned to the fast-paced action of the classic games with the 2016 game Doom and the 2020 sequel Doom Eternal.