The show stars Connie Britton and Dylan McDermott as a couple who move into a new home after a series of traumas. They soon discover that the property is haunted.
Heroes star Zachary Quinto recently signed up to play the house's former owner.
Sears has now joined the cast and will play the partner of Quinto's character, Deadline reports.
Quinto and Sears are expected to make their debut in the show's two-part Halloween episode.
Sears recently appeared in Torchwood and has also had roles in The Defenders and Raising The Bar.
Other stars who will appear in American Horror Story, which premieres on FX on October 5, include Lily Rabe, Eric Close, Jessica Lange, Shelby Young, Taissa Farmiga, Evan Peters, Denis O'Hare and Jamie Harris.
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Watch a trailer for American Horror Story below: