Titan Quest is a 2006 action role-playing game developed by Iron Lore Entertainment and published by THQ for Windows, first physically and then in 2007 through Steam. A mobile port was developed by DotEmu and published in 2016, and versions for PlayStation 4, Xbox One and Nintendo Switch were released in 2018. All these versions were published by THQ Nordic. The story follows a player-created protagonist as they navigate Ancient Greece, Egypt and China on a quest to defeat the Titans after they escape from their ancient prison. The gameplay is similar to the Diablo series, with player navigation being handled with a mouse-driven tile-based interface, and gameplay revolving around role-playing mechanics and real-time combat. Four expansions have been created for the game; Titan Quest: Immortal Throne in 2007, and three others between 2017 and 2021.

Titan Quest was envisioned by game designer Brian Sullivan as a role-playing game set in Ancient Greece similar to Age of Mythology. Production began in 2004 after a successful pitch to THQ. The script was written by Randall Wallace, while Sullivan acted as the designer. Despite being in a mythical setting, the team wanted to make the environments and towns feel as realistic as possible, leading to a large amount of research into ancient cultures. Enemies were inspired by the game's regional mythologies, with designs inspired by the stop-motion work of Ray Harryhausen. The music, composed by Scott Morton and Michael Verrette, was created to avoid the looping tracks of other games.

First announced in 2005, Titan Quest received generally positive reviews when released, being both praised as a good example of the genre while at the same time being criticized for its traditional gameplay. Sales of the main game and its expansion have been estimated as approaching one million units. The mobile port was tricky for its developers due to adapting the game for touchscreen controls: like the console version, it received positive reviews upon release. The engine and gameplay of Titan Quest later became the foundation for Grim Dawn, a video game developed by team members from Iron Lore following the studio's closure.

A sequel, titled Titan Quest II, was announced on August 11, 2023.

Titan Quest is an action role-playing game set in the pre-Roman Ancient World: these include Ancient Greece, Egypt, and the Silk Road leading through Asia. Players take control of an avatar: players are able to choose gender, name and tunic color. The three-dimensional world is navigated through an overhead third-person view, with the player character being controlled with the mouse through a point-and-click interface, while abilities are mapped to keyboard buttons. The environment is obscured by a "Fog of War" effect which blacks out unexplored environments on the minimap. Optional text tutorials for gameplay elements are unlocked progressively throughout the game and can be viewed at any time.

As they progress, players gain experience points through defeating enemies and completing quests for non-player characters (NPCs) scattered around environments: these raise a character's experience level, which grant access to fresh skills and points that can be used to upgrade character attributes such as health and energy levels, dexterity, intelligence, or strength. If the player dies, they respawn at rebirth fountains scattered through the world, although they lose some accumulated experience points. Quests are divided into Main Quests related to the central narrative, and side quests unique to particular areas of the world. Other NPCs can be found in towns and cities that act as Merchants selling equipment and items: these can be both bought and sold. Player characters have multiple equipment slots, which can take armor for limbs and torso, weapons or shields, and accessories that grant passive boons.

Fighting takes the form of real-time hack and slash combat, with players attacking randomly generated enemies highlighted by the mouse. Available weapon types for characters include swords, clubs, axes, and staves. In addition to the standard attack with an assigned weapon, offensive skills can be deployed. Using active skills triggers a cooldown meter, rendering that skill unusable until the meter depletes. Items and equipment can also be looted from fallen enemies and chests scattered through environments: these range in quality, with grey standing for damaged or low-quality gear while purple stands for a "Legendary" item, and orange denotes a Relic or Charm which can be equipped to the player to increase an attribute such as elemental damage resistance. The majority of items and equipment are randomly generated, and are associated with particular types of enemies. The world's currency, Gold, can be gained through quest completion, opening chests and defeating certain enemies. Gold is used with the various shop NPCs in exchange for their services.

After leveling up for the first time, the player can access Masteries, skill tree-based upgrade systems where skill points unlocked upon leveling up can be used to access and boost different skills. Mastery abilities expend energy, which is replenished over time when skills are not in use or by using energy drinks. There are eight available Masteries to choose from (Defence, Warfare, Hunting, Rogue, Earth, Storm, Nature and Spirit). Players can access two Masteries at any one time, mixing skills from both trees. The combination of different Masteries create different character classes: for instance, combining Nature and Earth Masteries grants players the "Summoner" class, while Defense and Warfare Masteries create the "Conqueror". There are 36 possible Classes, which include pure disciplines within one Mastery and hybrids between different Masteries. After a certain point in the game, the player has access to Mystics, NPCs which can reallocate skill points for a fee.