Priestess of Avalon is a 2000 novel by American writer Marion Zimmer Bradley, completed posthumously by Diana L. Paxson. It follows detailing the life of Helena, first wife of Western Roman Emperor Constantius Chlorus and mother of Constantine.

Marion Eleanor Zimmer Bradley was an American author of fantasy, historical fantasy, science fiction, and science fantasy novels, and is best known for the Arthurian fiction novel The Mists of Avalon and the Darkover series. Noted for the feminist perspective in her writing, her reputation has been posthumously marred by her daughter Moira Greyland's accusations of child sexual abuse, and for allegedly assisting her second husband, convicted child abuser Walter Breen, in sexually abusing multiple unrelated children.