Deus Ex is a series of action role-playing video games, originally developed by Ion Storm (2000-2003) and later Eidos-Montréal (2011-2017). The series began with the titular first game published by Eidos Interactive in 2000, and continued in three further mainline entries, and multiple spin-off titles. The gameplay combines first-person shooter, stealth, and role-playing elements, and has levels which offer multiple solutions to problems. A recurring feature is augmentations, mechanical or nanotechnological artificial organs which the protagonists use in their quests and impact gameplay styles. Set in a science fiction cyberpunk future, the series follows characters investigating mysteries tied to conspiracy theories and organizations including the Illuminati. Recurring themes across the series include transhumanism, terrorism, and discrimination.

The original game was created by Warren Spector, who wrote the original design document and created the initial setting inspired by multiple pieces of science fiction and political fiction. Ion Storm developed a sequel, Invisible War (2003), and attempted to make a third entry before closing down in 2005. The series was restarted by Eidos-Montréal in 2007, with the prequel Human Revolution (2011). The success of Human Revolution prompted a series revival, with a 2016 sequel and multiple spin-offs under then-publisher Square Enix.

The series as a whole has been praised for its gameplay design and setting, with some journalists citing the original Deus Ex as a prominent example of the "immersive sim", a category also attached to other series entries. Individual games have seen varying contemporary and retrospective reception. Beginning with Human Revolution, the series has seen multimedia expansions into comics and novels, many released as part of a project dubbed the "Deus Ex Universe".

A mobile spin-off set within the timeline of Human Revolution, Deus Ex: The Fall, was developed by N-Fusion Interactive and published in 2013 for mobile devices and 2014 for Windows. A sequel to The Fall was planned, but never entered production and left its story incomplete. A puzzle game for mobile platforms, Deus Ex Go, was developed by Square Enix Montreal and released in 2016. Two products related to Mankind Divided released in 2017; Breach, a standalone version of the game's multiplayer element, and a non-interactive virtual reality experience exploring some of the game environments.

Human Revolution had multiple media expansions released around it, expanding on its world and characters; these included a prequel comic series, and two prequel novels. Following Human Revolution, future entries and related media within the Deus Ex series was grouped under an umbrella project dubbed the "Deus Ex Universe". Mankind Divided also had several comics and a spin-off novel that filled in the narrative following Human Revolution.

All entries in the Deus Ex series are action role-playing video games which combine first-person shooter, stealth, and role-playing elements. The initial design of Deus Ex was inspired by multiple contemporary titles including Half-Life, Fallout, Thief: The Dark Project, and GoldenEye 007. The concept was for a "genre-busting" title which combined elements of different genres into a single title. The gameplay style has remained generally consistent, with later entries simplifying for ease of play, or adjusting and expanding elements including perspective and ability usage. The series has been grouped into the category of "immersive sim", a type of game associated with player freedom and realistic world design.

The games are played primarily from a first-person perspective, with games after Human Revolution featuring contextual switches to a third-person view. The original core concept for the gameplay of Deus Ex was for the player to have great freedom for completing objectives and progressing the game, and in later entries freedom of choice has remained an important element. Players take on the role of the player character while exploring hub areas which contain main and side quests, and objectives which encourage a number of different gameplay approaches, lethal and non-lethal combat options, and multiple possible story outcomes. Hacking into devices such as computers and locks is a recurring feature across the series. Individual entries have unique gameplay elements such as the universal ammunition in Invisible War, and the multiplayer Breach mode in Mankind Divided.

A recurring story and gameplay feature across the series is "Augmentations", artificial organs which use either machinery or nanotechnology; these augmentations range from permanent features such as radio links, to optional abilities including cloaking, hacking features, and different combat-focused elements. The original Deus Ex uses both augmentations, and a skill point system which dictates further character proficiencies. Invisible War ties augmentations to different categories which are unlocked and upgraded with items called Biomods. Augmentations from Human Revolution onwards are unlocked using items called Praxis Kits, which can be bought with in-game currency, awarded for completing quests, or unlocked by gathering experience points. Human Revolution had augmentations geared more towards action or stealth-based approaches, with Mankind Divided adjusting augmentations to be viable across more playstyles.