- Aludran Reptiloid: Highlander's Bride
- Beheaded Rocketeer
- Beheaded Bomber
- Beheaded Firecracker
- Beheaded Kamikaze
- Common Aludran Reptiloid
- Cucurbito the Pumpkin
- Exotech Larva (Boss)
- Fiendian Reptiloid Demon
- Highlander Aludran Reptiloid
- Kleer Skeleton
- Kukulkán, the Wind God (Boss)
- Lava Golem
- Major Bio-mechanoid
- Marsh Hopper
- Minor Bio-mechanoid
- Mordekai the Summoner (Boss)
- Reeban Electro-Fish
- Scythian Witch-Harpy
- Sirian Werebull
- Ugh-Zan III (Boss)
- Zorg Commander
- Zorg Mercenary
- 12 Gauge Pump-Action Shotgun
- Double Barrel Coach Gun
- M1-A2 Thompson Submachine Gun
- Military Knife
- MK III Grenade Launcher
- P-LAH Chainsaw
- RAPTOR Sniper Rifle
- Rotating Cannon
- SBC Cannon
- Schofield .45
- Serious Bomb
- Static Cannon
- XL2 Lasergun
- XM214-A Minigun
- XOP Flamethrower
- XPML21 Rocket Launcher
Gameplay-wise, Serious Sam: Xbox is essentially the same as both Serious Sam: The First Encounter and Serious Sam: The Second Encounter, though there have been many changes and modifications. There are also several features which are otherwise not present in Serious Sam 1.
For instance, there is a "multikill" system, which multiplies your score if you kill multiple enemies with a single shot. (x1 multiplier for each enemy killed) The more multikills you do in a short period of time, the higher the multiplier. Other features include a life system, which re-spawns you in place when you die, and auto-aiming. The player can access multiplayer via system link and split-screen.
The player can also align their favorite weapons to the X and Y button by selecting a weapon and holding X or Y, then press X or Y again to select that weapon.
- Sam now has a new model, and no shades. He is a lot less like Duke Nukem, like he was in Serious Sam 1.
- Beheaded Bombers, Firecrackers, Kamikazes and Rocketeers look more muscular than in Serious Sam 1, and they have less human-like heads (except the Kamikaze, who has no head), without a mouth and with glowing light blue eyes and they all have a belt with a skull on it. The Kamikaze's back also looks different.
- Cucurbitos have fingers and a different chainsaw. Ugh-Zan III also has fingers.
- Several new introductory levels are added, giving a truncated origin story for Sam of his fights against Mental's forces on present-day Earth leading up to his use of the Time-Lock.
- Some levels are split into two parts, such as:
- Karnak is split up into Karnak and Ankh Pool.
- Sierra de Chiapas is split up into Sierra de Chiapas and Catacombs.
- The City of the Gods is split up into The City of the Gods and Inner Sanctum.
- Ziggurat is split up into Ziggurat and Gates of Persepolis.
- The Courtyards of Gilgamesh are split into Courtyards of Gilgamesh and Temple of Gilgamesh.
- Tower of Babel is split up into Ceremonial Halls and Tower of Babel.
- The Citadel is split up into The Bridge and The Citadel.
- Land of the Damned is split up into Winter Wonderland and Lava Caves.
- The Grand Cathedral is split up into Corridor of Death and The Grand Cathedral.
- Certain levels such as City of the Gods and The Elephant Atrium are missing significant portions.
- Deathmatch maps Compound, Desert Temple, Red Station and We Got Skulls'n'Bones Too from the PC version have been removed. They have been replaced by 5 new maps: Ashes to Ashes, Brkeen Chevap, Docking Bay, Hoodlum's Backyard and Sun Palace.
- There are some new cinematics, and some different cinematics, such as the intro and The Second Encounter intro is skipped to where Sam flies aboard the SSS Centerprice. Specifically, Oasis, Alley of the Sphinxes, The City of the Gods, Ziggurat, The Citadel, Land of the Damned and The Grand Cathedral have new intros, while the Temple of Hatshepsut, Tomb of Ramses III, Valley of the Kings, Dunes, Suburbs, Sewers, Metropolis, Karnak (or "Ankh Pool", here), Luxor, The Pit, Tower of Babel and The Citadel had new outros.
- There is a time-lock in the Ziggurat and The Citadel, which serve as "exit" time locks. "Entry" time-locks look like doorways.
- The coach gun, grenade launcher, knife, Schofield .45 and Thompson have different models. The serious bomb also has a different model, and is required to be equipped first, then used, unlike the Serious Sam 1 version, where it is instantly used.
- There are serious bombs scattered around The First Encounter levels, and there's a chainsaw in Temple of Hatshepsut.
- The player can pick up ammo, health and armor items even if you the maximum amount allowed for that item, unlike the PC version.