The British actress will become a series regular when the HBO drama returns, TV Line reports.
She will play Nora, the "sister" of Eric (Alexander Skarsgård), both of whom were turned into vampires by Godric.
Nora is said to care about Eric a lot, but she has focused all her energies on working as a double agent in the Vampire Authority. The character has also been described as clever, "intimidating", and deceitful.
Griffiths is probably best known for her role in the British drama Robin Hood and has also appeared in UK shows such as The Little House, Collision and U Be Dead.
She recently worked on the CW pilot Awakening, but the project was not picked up.
The fifth season of True Blood is due to premiere on HBO next summer.
> Stephen Moyer confident of True Blood future
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