Atwell will play Peggy Carter in The First Avenger: Captain America.
The British actress joins Chris Evans and Sebastian Stan in the film, directed by Joe Johnston.
According to a news release from Marvel, the character of Peggy fell in love with Captain America while they fought together in the war effort.
The movie will focus on the "early days" of the Marvel Comics universe and feature Steve Rogers volunteering for an experimental Super Soldier programme that turns him into Captain America.
The picture arrives in US cinemas on July 22, 2011.