The Veronica Mars actress, who most recently guest-starred on Bones, will be playing a member of the Vampire Authority's staff on the fifth season of the HBO series, TV Line reports.
Majorino will first be seen on the new run's third episode, which airs June 24.
Stephen Moyer, who plays Bill on the series, recently revealed that Eric (Alexander Skarsgård) will be forming an alliance with his character.
Earlier this month, executive producer Mark Hudis was named as Alan Ball's successor for showrunner if True Blood receives a sixth season. Ball stepped down from his duties as showrunner in February.
True Blood returns June 10 at 9/8c on HBO.
Watch a trailer for the new season of True Blood below: