House of the Dragon will be set about 300 years before events in the original series, which ended its eight-season run earlier this year.
The announcement came hours after another planned prequel, due to star Naomi Watts, was reportedly cancelled.
Game of Thrones was HBO’s most popular series. It won multiple Emmy awards and drew tens of millions of viewers.
“It’s my pleasure to announce today that we are ordering House of the Dragon straight to series for HBO,” said Casey Bloys, president of HBO programming, at a launch event in Los Angeles for the HBO Max streaming platform.
“It tells the story of House Targaryen and the early days of Westeros.”
In a tweet, Games of Thrones said House of the Dragon would be co-produced by Game of Thrones author George RR Martin, Ryan Condal and Miguel Sapochnik.
#HouseOfTheDragon, a #GameofThrones prequel is coming to @HBO.
The series is co-created by @GRRMSpeaking and Ryan Condal. Miguel Sapochnik will partner with Condal as showrunner and will direct the pilot and additional episodes. Condal will be writing the series. pic.twitter.com/9ttMzElgXm
— Game of Thrones (@GameOfThrones) October 29, 2019
A pilot episode for an unnamed prequel starring Oscar nominee Naomi Watts was filmed over the summer.
Earlier on Tuesday, sources quoted by The Hollywood Reporter said executive producer Jane Goldman had emailed the cast telling them that HBO had cancelled the show.
The reasons behind the decision were not immediately clear and HBO did not comment.
However, entertainment website Deadline reported that the pilot had not been well received by HBO.
That prequel, also co-created by George RR Martin, was reported to have been set about 5,000 years before events in the original series.
On Monday, Game of Thrones writers David Benioff and DB Weiss announced that they had left the next Star Wars trilogy, due to launch in 2022. The news came after the pair signed an exclusive deal with Netflix.
Sky have extended their broadcast deal with HBO for another five years, according to Deadline, which means that the prequel is likely to air on Sky Atlantic in the UK – as Game of Thrones did.