The actress - who played Maria Hill in The Avengers - has been listed among the cast members of the sequel by Variety.
Smulders, who has not been mentioned in connection to the film previously, was unceremoniously included in an article confirming Frank Grillo in the role of Crossbones.
S.H.I.E.L.D. - the defence organisation of which Hill is a member - is expected to play a major role in the 2014 Marvel Studios movie, which will be a political thriller based on Ed Brubaker's 'Winter Soldier' storyline.
The search for an actress to play Sharon Carter is continuing, with Downton Abbey's Jessica Brown Findlay thought to be the frontrunner.
Smulders has previously revealed that she has signed on to appear in eight Marvel films. Captain America 2 will be her second when it arrives on April 4, 2014.
Watch a trailer for the first Captain America movie below: