Janina Gavankar, who previously starred in The L Word and The Gates, will become a series regular on the show.
She will play Luna, a schoolteacher and shapeshifter who attends a gathering with similar people.
Dirt and Life Unexpected star Alexandra Breckenridge has landed a recurring role as Daisy. Daisy is a member of the Wiccan group founded by Marnie (Fiona Shaw).
Vedette Lim, who has guest starred in shows including Parenthood, Medium and As The World Turns, has also signed up for a recurring role. She will play Naomi, who takes part in underground cagefights to earn money.
Meanwhile, Jessica Tuck has been promoted to a series regular. Tuck joined the show in the first season as Nan Flannigan but her role has grown in recent episodes and she has now been made into a full cast member.
Production on the fourth season of True Blood is expected to begin next week.