A weapon is used by the player to attack opponents. It is displayed onscreen as part of the body of the player character to simulate a first person perspective. In Doom it is the main aspect of the game action.
Each appears in the form of an item that, when picked up, provides the player with an additional weapon with which to attack opponents, as well as a bit of ammo for the corresponding weapon. Normally weapons are either placed in levels by the designers or dropped by dying opponents (such as zombies in the Doom games). Once a weapon is picked up during a game any additional weapons of the same type will count only as ammunition.
Each weapon has advantages and disadvantages when used against a given monster. Broadly speaking, however, the larger weapons do more damage but are harder to find and harder to keep stocked with ammunition. In each game, one or two weak weapons require no ammo and are therefore always available.
No matter which weapon a player is wielding, his sprite (in multiplayer games, or source ports with a "chasecam" mode) is perpetually shown to be armed with the same generic assault rifle as the former humans; a weapon similar to this, although presumably intended to be the chaingun, appears on the game's title screen.
Picking up a weapon plays the sound effect DSWPNUP.
Early alpha releases of Doom armed the player with a semi-automatic assault rifle, optionally equipped with a double-bladed bayonet as a melee weapon.  This weapon was not included in the finished game. The rifle sprites revealed the player character to be wearing elbow pads, which can still be seen on the player character sprite along with the rifle itself. A few other weapons were also included in the alpha versions.
- Fists: Extremely basic close-range weapon. Never runs out of ammo, but only about as powerful as a pistol shot; normally used only as a last resort or with a berserk powerup.
- Chainsaw: Does damage like the normal fist, but four times faster.
- Pistol: The default long-range weapon. Not too effective.
- Shotgun: A good general-purpose weapon, especially at close range. Reload time is slightly longer than normal.
- Chaingun: Very good against large crowds, but its rapid rate of fire can quickly deplete its ammo supply.
- Rocket launcher: Fires explosive rockets. Does a lot of damage, but can also seriously hurt the player if used indiscriminately at close range.
- Plasma gun: Shoots pulses of blue-hot plasma at high speed, which can take down groups of incoming enemies easily — if aimed properly.
- BFG9000: The "Big Fucking Gun." Somewhat counter-intuitive to operate at first, but kills almost any monster in one shot.
The chaingun and pistol both use the same ammo (bullets), so using either will also affect the other in ammo. The shotgun and super shotgun (see below) also share their ammo pool (shells) as do the plasma gun and BFG9000, which both use cells.
All of the above weapons, with the addition of:
- Super shotgun: A double-barrelled, sawed-off shotgun which takes even longer to reload, but at close range is even more deadly than the regular shotgun.
All of the above weapons, with the addition of:
- Unmaker: A demonic laser gun which is a plasma gun clone at first but can be upgraded in firepower by the collection of Demon Keys.
All of Doom's weapons are present in some form in Doom 3, with the addition of:
- Flashlight: Not a weapon per se, but it doubles as a crude club.
- Machine gun: Similar to the chaingun, but not as powerful (but more accurate).
- Grenades: Standard thrown fragmentation grenades on a timer.
- Soul cube: Charged by the killing of monsters, and then released as a powerful seeking weapon that transfers health from the monster it hits to the player.
The Doom 3: Resurrection of Evil expansion pack contains all of Doom 3's weapons except the Soul Cube and the Chainsaw, but introduces a version of the gravity gun called the Grabber, and includes the super shotgun (as in Doom II). It also introduces an artifact somewhat like the Soul Cube.
The 2016 Doom reboot contains all of the Doom II weapons and numerous additions. Some of the weapons are multiplayer-only, and some of these are additionally only available if the corresponding DLC packs are owned.
- UAC EMG sidearm: The starting weapon for the player, replacing the original conventional pistol. It can be charged for a more powerful attack, but is very weak. Being energy based, it has infinite ammunition.
- Combat shotgun
- Gauss cannon
- Heavy assault rifle: An assault rifle taking the place of Doom 3's machine gun. It has upgrades for mini missiles and a scope.
- Plasma rifle
- Rocket launcher
- Super shotgun
- Burst rifle
- Lightning gun (also available in SnapMap)
- Reaper (requires Hell Followed DLC)
- Static cannon
- EMG Mark V pistol (requires Unto the Evil DLC; also available in SnapMap)
- Vortex rifle
- Gauntlets of the Necromancer
- Elven Wand
- Ethereal Crossbow
- Dragon Claw
- Phoenix Rod
- Punch Dagger
- Assault Gun
- Mini-Missile Launcher
- Grenade Launcher
- The Sigil of the One God