Author George RR Martin created the new series alongside screenwriter Jane Goldman.
The channel has ordered a pilot episode for the show, set thousands of years before the battles over the Iron Throne.
HBO executives say any spinoff will not be broadcast until after Game of Thrones‘ final season in 2019.
The channel said in statement if picked up, the prequel will chronicle “the world’s descent from the golden Age of Heroes into its darkest hour”.
“From the horrifying secrets of Westeros’ history to the true origin of the White Walkers, the mysteries of the East, to the Starks of legend… it’s not the story we think we know.”
British author Jane Goldman, who wrote Kick-Ass, X-Men: First Class and the Kingsman films, will serve as a showrunner for the as-yet-unnamed show.
Game of Thrones is HBO’s most popular series to date. It has won multiple Emmy awards and drawn tens of millions of viewers worldwide.
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