Dota 2 is a free-to-play multiplayer online battle arena (MOBA) video game developed and published by Valve Corporation. The game is the stand-alone sequel to Defense of the Ancients (DotA), a mod for the 2002 video game Warcraft III: Reign of Chaos and its expansion pack, The Frozen Throne. Dota 2 was released for Microsoft Windows, OS X, and Linux in July 2013, following a Windows-only public beta testing phase that began in 2011, and is one of the most actively played games on Steam, with maximum peaks of over a million concurrent players.

Dota 2 is played in matches between two five-player teams, each of which occupies a base in a corner of the playing field. Each player controls one of 111 playable characters, called "heroes", that feature unique abilities and styles of play. During a match, the player collects gold, items, and experience points for their hero, while fighting heroes of the opposite team. A team wins by destroying the other side's "Ancient" building, located within the opposing base.

Development of Dota 2 began in 2009 when IceFrog, lead designer of the original DotA mod, was hired by Valve for the same role. Upon release, Dota 2 was praised by critics for its gameplay, production quality, and faithfulness to its predecessor, despite being criticized for its steep learning curve. Dota 2 initially used the original Source game engine until it was ported over to Source 2 in September 2015, making it the first game to use it. The game also allows for the community to create custom game modes, maps, and cosmetics for the heroes, which are then uploaded to the Steam Workshop. The popularity of Dota 2 has led to official merchandise being produced for it, including apparel, accessories, toys, and promotional tie-ins to other games and media.

                                              Dota 2 has a widespread and active eSports scene, with teams from across the world playing in various leagues and tournaments. Premium Dota 2 tournaments often have prize pools totaling millions of dollars, the highest of any eSport. The largest of them is known as The International, which is hosted by Valve and takes place annually at the KeyArena in Seattle. Starting in 2015, Valve also began sponsoring smaller, but seasonally held tournaments known as the Major Championships, the first of which was held in Frankfurt, Germany. Dedicated media coverage of professional tournaments are broadcast live on the internet, and sometimes on television networks.