The game will charge users for new items and apparel to customise their characters.
The announcement is accompanied by the launch of the official DOTA 2 store, which will be open for business prior to the game's launch.
It will sell user-generated content and items created by Valve, which can be used immediately and will be carried over once the game is released.
Users not involved in beta testing will receive a code for immediate access if they purchase the Early Access Bundle of items.
"The opening of the Dota Store is a big part of our final push to launch," revealed DOTA 2 designer IceFrog.
"With support for the Steam Workshop, the majority of the items made available on day one were created and being sold by members of the community.
"By making the game Free to Play, we hope to give gamers the ability to decide how - and how much - they want to invest in the game."
Valve also revealed plans to host the International Dota 2 Championships 2012 in Seattle later this year.
Valve previously hinted at a 2011 release date for DOTA 2 during an interview with Digital Spy at Gamescom.
The developer has since confirmed that the game will be released in its current form, following a shorter beta testing phase.
It is expected to launch later this year.