The drama - based on Stephen King novel The Colorado Kid - follows a former FBI agent (Emily Rose) who investigates the paranormal secrets of a small town.
One Life to Live star Williamson will appear as a series regular in the third season, Haven story editors Lilla and Nora Zuckerman revealed.
The pair told WormholeRiders.net's BlogTalk Radio podcast that her character Claire is "Haven's resident shrink".
In addition to her role as Jessica Brennan on ABC's One Life to Live, Williamson has previously appeared in episodes of Gossip Girl and Law & Order: Criminal Intent.
Haven was officially picked up for a third season in October of last year.
The new 13-episode run will premiere on Syfy in late 2012.