Halo 3, one of the most anticipated videogames for the Xbox 360, will be released to the public in test, or beta, from next year.
Halo 3 is an Xbox 360 exclusive
The online multiplayer element of the game will be made available to the public with feedback going to developers Bungie.
More than 14.5m copies of Halo titles have been sold and more than 800m hours of online Halo games have been played.
The announcement comes in the week Sony and Nintendo release new consoles.
"We can confirm that our fans will have an opportunity to play Halo 3 multiplayer over Xbox Live before the final game is available," wrote community manager Brian Jarrard on the Bungie website.
The decision to release the multiplayer as a public beta marks a significant departure for Bungie, which is traditionally highly secretive when it comes to releasing details about its games.
Bungie has also said that new online multiplayer maps for Halo 2 would only be available to Xbox 360 owners and not players of the first Xbox.
That decision is likely to anger many Xbox owners, a fact acknowledged by Bungie who made clear the choice was made by Microsoft.
"We can't really blame you for potentially being a little upset at this news," wrote Mr Jarrard.
"We have the opportunity to give our fans some new content over two years after the game shipped but ultimately, when it comes to pricing and timing and distribution and exclusivity, it's not up to Bungie."
Halo is the Xbox's most valuable franchise and Microsoft will be hoping that releasing an unfinished element of Halo 3 to the public and new maps for Halo 2 only on the Xbox 360 will keep gamers loyal to the platform.