Who would be playing the Mad Titan in the film, where he’ll be fully CG and would just need a voice, has been the subject of debate among fanboys for months. Many believed that Vin Diesel would voice both Groot and Thanos, as such a thing isn’t uncommon in movies like this.
According to a report on Latino Review, Josh Brolin will be providing the voice to Thanos. As it’s a character that will most definitely have a bigger role in the future of the Marvel Cinematic Universe, he’s a pretty big name to secure for the character.
As of right now Marvel hasn’t confirmed it, but Variety’s sources have so it appears to be official.