In which year was Dota 2 officially released?

Dota 2 was officially released in 2013.

Valve launched the game on Steam for Windows on July 9, 2013, after nearly two years of testing. Versions for OS X and Linux followed on July 18. This formal release came after the game had already attracted substantial attention through its beta and competitive debut.

Dota 2 was publicly shown at Gamescom in 2011, where Valve also hosted the first International tournament. Closed beta invitations began around that event, and the game later moved into open beta in September 2011. Those earlier dates can make the release year seem confusing, but they describe testing and public availability rather than the official launch.

The game grew out of Defense of the Ancients, a popular Warcraft III mod. Valve hired IceFrog, one of DotA’s lead developers, in 2009 to help create a standalone version. Dota 2 initially launched without every hero from the original mod; the remaining heroes arrived through later updates.