Deus Ex is a 2000 action role-playing game developed by Ion Storm and published by Eidos Interactive. Originally released for Microsoft Windows, it was released for Mac OS the same year, and for PlayStation 2 in 2002. A remaster for modern platforms is set for release in 2026. The gameplay—combining first-person shooter, stealth, and role-playing elements—features exploration and combat in environments connected to multiple city-based levels, with quests that can be completed in a number of ways and character customization based around cybernetic enhancements. Conversations between characters feature a variety of responses, with choices at key story points affecting how some events play out. A post-release patch incorporated deathmatch-style multiplayer.

Deus Ex is set in 2052, in a dystopian cyberpunk future beset by terrorist acts, economic inequality, and a plague dubbed the Gray Death. The player character, the cybernetically enhanced JC Denton, is an anti-terrorism agent who is deployed when a terrorist group interrupts supplies of a rare Gray Death vaccine. Investigating the incident, Denton ends up involved in a struggle between multiple factions for control of the world. The story is inspired by popular conspiracy theory motifs, incorporating groups including the Illuminati and Majestic 12.

The game was created by Warren Spector, who acted as director and producer, and put together a design concept during the early 1990s under the title "Troubleshooter". After being approached by Ion Storm about creating a project with complete creative freedom, Spector began pre-production in 1997. Staff included lead designer Harvey Smith, lead writer Sheldon Pacotti, and lead composer Alexander Brandon. The game was built using the Unreal Engine, which led to issues with coding and non-playable character behavior. Due to technical and time limitations, some planned features and areas had to be downscaled or cut entirely.

Upon release, Deus Ex was a commercial success, selling one million copies worldwide. It saw critical acclaim from game journalists for its design and freedom of player choice. Its graphics saw more mixed reactions, and the voice acting was faulted. The PlayStation 2 port saw mixed reactions, but many praised its adaptation of the game's mechanics to console. It won multiple gaming awards, has been ranked among the best video games of all time, and fostered an active fan community. It has also been cited as a prominent example of the "immersive sim" genre. Deus Ex was expanded into a series of the same name, with a sequel, Deus Ex: Invisible War, releasing in 2003.

Deus Ex is an action role-playing game with incorporated first-person shooter and stealth mechanics. Players take on the role of JC Denton, a man equipped with nanotechnology-based cybernetic enhancements called augmentations. At the game's opening the player chooses a combat difficulty−"Easy", "Medium", "Hard", or "Realistic"−and customize Denton's real name, visual appearance, and starting skills. The player moves around the environment, being able to climb ladders and jump onto crates, swim through bodies of water, and crouch to negotiate narrow spaces. The environment also has interactable objects such as computers and keypads, and certain objects in the game can be carried or thrown, from small items like boxes to human bodies.

Each hub area of the game has a variety of missions given to the player by non-playable characters (NPCs), which the player can choose to accept or ignore; these missions include quests linked to the main story and side quests unique to each hub area. These objectives can be completed in a variety of ways; these include using stealth to infiltrate an area, opening access points using hacking, launching an armed assault, or a combination of different tactics. When talking with NPCs, the player has access to multiple dialogue options, with the option chosen influencing the course of the conversation. Short messages are also given which direct the player towards different objectives.

There are a variety of melee and ranged weapons in the game, both lethal and non-lethal; ranged weapons can be upgraded using kits found during gameplay. Items, weapons, ammunition, and equipment can be purchased with credits, the in-game currency either gathered in the environment or earned through quests, or found during exploration. Items that can be picked up include weapon ammunition, health restoratives, and tools for bypassing barriers; lockpicks for mechanical locks, and multitools for electronic devices. The player's health is divided between multiple body parts, each requiring healing with items or at healing stations. Inventory space is limited, with items and weapons taking up varying amounts of space.

Denton's abilities can be enhanced over the course of the game. Skill points earned by completing quests can be spent on the eleven skill types; computers, electronics, the environment, lockpicking, medicine, swimming and five weapon proficiencies. For instance, raising environment proficiency allows for higher jumps and more resistance to hostile environmental effects. During character creation skill points can be reassigned from one skill to another, but after this point skill increases are permanent. Additionally, augmentation canisters can be found or bought throughout the game. Denton starts with three augmentations; the story-based infolink communicator, a light, and a tool for identifying hostiles. Other augmentations, such as increased movement speed, can be unlocked and upgraded using canisters a maximum of four times. One body area can only have one augmentation installed, and they are permanent. Using an augmentation costs energy, which is restored using consumable biocells.