Doom is a 1993 first-person shooter game developed and published by id Software for MS-DOS. It is the first installment in the Doom franchise. The player assumes the role of a space marine, later unofficially referred to as Doomguy, fighting through hordes of undead humans and invading demons. The game begins on the moons of Mars and finishes in hell, with the player traversing each level to find its exit or defeat its final boss. It is an early example of 3D graphics in video games, and has enemies and objects as 2D images, a technique sometimes referred to as 2.5D graphics.

Doom was the third major independent release by id Software, after Commander Keen (1990–1991) and Wolfenstein 3D (1992). In May 1992, id started developing a darker game focused on fighting demons with technology, using a new 3D game engine from the lead programmer, John Carmack. The designer Tom Hall initially wrote a science fiction plot, but he and most of the story were removed from the project, with the final game featuring an action-heavy design by John Romero and Sandy Petersen. Id published Doom as a set of three episodes under the shareware model, marketing the full game by releasing the first episode free. A retail version with an additional episode was published in 1995 by GT Interactive as The Ultimate Doom.

Doom was a critical and commercial success, earning a reputation as one of the best and most influential video games of all time. It sold an estimated 3.5 million copies by 1999, and up to 20 million people are estimated to have played it within two years of launch. It has been termed the "father" of first-person shooters and is regarded as one of the most important games in the genre. It has been cited by video game historians as shifting the direction and public perception of the medium as a whole, as well as sparking the rise of online games and communities. It led to an array of imitators and clones, as well as a robust modding scene and the birth of speedrunning as a community. Its high level of graphic violence led to controversy from a range of groups. Doom has been ported to a variety of platforms both officially and unofficially and has been followed by several games in the series, including Doom II (1994), Doom 64 (1997), Doom 3 (2004), Doom (2016), Doom Eternal (2020), and Doom: The Dark Ages (2025), as well as the films Doom (2005) and Doom: Annihilation (2019).

Doom is a first-person shooter presented with 3D graphics. While the environment is shown in a 3D perspective, the enemies and objects are instead 2D sprites rendered at fixed angles, a technique sometimes referred to as 2.5D graphics or billboarding. In the single-player campaign mode, the player controls an unnamed space marine—later unofficially termed "Doomguy"—through military bases on the moons of Mars and in hell. To finish a level, the player must traverse through labyrinthine areas to reach a marked exit room. Levels are grouped into named episodes, with the final level of each focusing on a boss fight.

While traversing the levels, the player must fight a variety of enemies, including demons and possessed undead humans. Enemies often appear in large groups. The five difficulty levels adjust the number of enemies and amount of damage they do, with enemies moving and attacking faster than normal on the hardest difficulty setting. The monsters have simple behavior: they move toward their opponent if they see or hear them, and attack by biting, clawing, or using magic abilities such as fireballs.

The player must manage supplies of ammunition, health, and armor while traversing the levels. The player can find weapons and ammunition throughout the levels or can collect them from dead enemies, including a pistol, a shotgun, a chainsaw, a plasma rifle, and the BFG 9000. The player also encounters pits of toxic waste, ceilings that lower and crush objects, and locked doors requiring a collectable keycard or a remote switch. Power-ups include health or armor points, a mapping computer, partial invisibility, a radiation suit against toxic waste, invulnerability, or a super-strong melee berserker status. Cheat codes allow the player to unlock all weapons, walk through walls, or become invulnerable.

Two multiplayer modes are playable over a network: cooperative, in which two to four players team up to complete the main campaign, and deathmatch, in which two to four players compete to kill the other players' characters as many times as possible. Multiplayer was initially only playable over local networks, but a four-player online multiplayer mode was made available one year after launch through the DWANGO service.

Doom is divided into three episodes, each containing eight main levels: "Knee-Deep in the Dead", "The Shores of Hell", and "Inferno". A fourth episode, "Thy Flesh Consumed", was added in an expanded version, The Ultimate Doom, released two years after Doom. The campaign contains very few plot elements, with a minimal story presented mostly through the instruction manual and text descriptions between episodes.