Book author George RR Martin developed the new series alongside British screenwriter Jane Goldman.
HBO has ordered a pilot episode for the show, set thousands of years before the battles over the Iron Throne.
Executives say any spinoff will not be broadcast until after Game of Thrones’ final season in 2019.
If picked up, the prequel will chronicle “the world’s descent from the Golden Age of Heroes into its darkest hour”, HBO said in a statement.
“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.”
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.
The channel has teased what Mr Martin described as “successor shows” in the past, announcing in May 2017 a number of writers who were working on these series.
Other than Ms Goldman, they include Max Borenstein, Brian Helgeland, Carly Wray and Bryan Cogman.
Previous reports suggest HBO could be willing to throw more than $50m at each season of these new shows.
But HBO’s programming president Casey Bloys previously told Hollywood Reporter these series would not focus on the current storyline.