Gish will appear in the seventh episode of the show’s second season, according to Entertainment Weekly.
She will play Anita, the mysterious leader of a pack of humans who have the ability to transform into wolves, like series regular Red (Meghan Ory).
Gish is well known for her role as Monica Reyes on The X-Files as well as starring roles in The West Wing and Showtime drama Brotherhood.
Once Upon a Time returns to ABC in the US on 30th September. Channel 5 is expected to air the new season in early 2013.