Naomi Watts will have a starring role in the "Game of Thrones" prequel.
"Game of Thrones" is based on Martin's "A Song of Ice and Fire' book series.
The new show was the first of several potential "successor series" to be picked up to pilot.
It is set thousands of years before the events of "Game of Thrones," with Watts playing a charismatic socialite with a dark secret.
"Casting is now underway for The Long Night, the first of the Game of Thrones successor series to ordered to film," the author wrote.
"HBO has just announced the first cast member: Naomi Watts is coming on board as one of our stars."
It previously reported the prequel will begin filming in Belfast, Northern Ireland, in October.
"Game of Thrones" will return for an eighth and final season in 2019. The series stars Emilia Clarke, Kit Harington, Lena Headey, Sophie Turner, Maisie Williams and Peter Dinklage.
Watts is known for such films as "Mulholland Drive," "The Ring" and "Eastern Promises."
She also starred in the "Twin Peaks" revival and the Netflix series "Gypsy."