The appearance of Thanos in The Avengers 2 had been speculated since a mid-credits scene in the original film depicted the 'Mad Titan' conferring with the leader of the Chitauri.
Thanos creator Jim Starlin confirmed at Montreal Comic-Con that a deal is in place for his character to appear in both movies, according to Cosmic Book News.
The news comes after Starlin threatened Marvel with a lawsuit earlier in the year, saying that the company had used both his creations of the Infinity Gauntlet and Thanos in its films without paying him, even complaining that he had to "buy his own ticket" to the movie.
As yet it is unclear whether Thanos will be the main villain in both movies, but his status as one of Marvel's non-Earth-based characters appears to make him a good fit for the cosmic focus of the Guardians of the Galaxy film.
James Gunn was officially confirmed as director for Guardians of the Galaxy yesterday, and is currently working on re-writing the script.
In the Marvel universe, Thanos is a mutant from the moon Titan, and is descended from the superhuman race of Eternals. His servitude and love for Mistress Death leads to him raining death and destruction across the Marvel universe in an attempt to win her affection.
Guardians of the Galaxy is set for release on August 1, 2014, with The Avengers 2 set for a May 2015 release date.
Watch Thanos's mid-credits appearance in The Avengers below: