Sebastian Stan Expecting Baby With GF Annabelle Wallis

Sebastian Stan and Annabelle Wallis are expecting their first child together, TMZ has learned!!!

We're told the two actors -- who have been together for about four years -- are gearing up to welcome a baby, though details like the due date and gender of the child are still under wraps.

The timing comes during a particularly busy stretch for both stars ... Sebastian has a packed year ahead with the upcoming release of his film “Fjord” ... along with the highly anticipated Marvel installment “Avengers: Doomsday.”

Annabelle has a couple big projects on the horizon as well. She stars alongside Jason Statham in the action film “Mutiny” -- slated to hit theaters in August -- and she’s also set to appear in Netflix’s “Unabomb,” a project about the Unabomber, Ted Kaczynski.