The drama stars Life Unexpected's Britt Robertson as Cassie, a young girl who moves to Salem and discovers that she is a witch.
Harold has now signed up to play Thomas Meade, Deadline reports. Thomas is the father of Diane, one of Cassie's friends and the head of her coven.
Harold is currently starring as Julian in Hellcats. He previously played Jackson in Desperate Housewives and has also appeared in shows such as Queer As Folk USA, Deadwood and Vanished.
Secret Circle, which is based on books by The Vampire Diaries author LJ Smith, is being executive produced by The Vampire Diaries showrunner Kevin Williamson.
The cast also includes Natasha Henstridge, Louis Hunter, Thomas Dekker, Phoebe Tonkin, Shelley Hennig and Jessica Parker Kennedy.
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