The star - more famous for his roles onstage - has completed negotiations with Marvel bosses to helm the film, due for release in 2010.
Branagh's last big-screen directorial outing was 2007's Sleuth, starring Sir Michael Caine and Jude Law.
And he can't wait to take the challenge of directing a Hollywood blockbuster.
He says, "We're getting the story and visual effects together and all of that is very exciting."
Thor follows the tale of a disabled medical student whose alter ego is the Norse god.