The actress will appear in a forthcoming 'origins' episode of the ABC fantasy drama in the New Year, reports Deadline.
McGowan - who is best known for her starring role as Paige Matthews in supernatural drama Charmed - will play a younger version of Barbara Hershey's character Cora during her appearance on the show.
Her other credits include Nip/Tuck, The Black Dahlia and Grindhouse.
24 actor Gregory Itzin was recently tipped to appear in the twelfth episode of season two, playing Alphonse, the father of Dr Frankenstein (David Anders).
The episode will also feature Chad Michael Collins (Last Resort) as Frankenstein's brother Gerhardt.
Once Upon a Time will return to ABC with new episodes on Sunday, January 6, 2013.