David Benioff and DB Weiss will begin work on Star Wars when Game of Thrones ends in 2019, Lucasfilm announced.
Little is known about the project.
But it will develop a story separate from the current series, and also from a trilogy planned by director Rian Johnson, who was behind 2017's The Last Jedi.
"In the summer of 1977 we travelled to a galaxy far, far away, and we've been dreaming of it ever since," Benioff and Weiss said in a joint statement.
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Game of Thrones, known for its violence and nudity, is markedly different from the more family-friendly Star Wars films.
But Kathleen Kennedy, president of Lucasfilm, said the pair "are some of the best storytellers working today".
Some have criticised the announcement, pointing to a lack of diversity among key figures in the Star Wars franchise.
Since Disney purchased Lucasfilm, and the accompanying rights to Star Wars, it has ramped up production of new additions to the series.
This year sees the release of spin-off film Solo, the final part of the recent trilogy is due next year, while there are other TV projects planned.
It is not the first tie-up between Game of Thrones and Star Wars.
Gwendoline Christie - who plays Brienne of Tarth in the TV franchise - took on the role of Captain Phasma in both The Force Awakens and its sequel, The Last Jedi.
Her Game of Thrones co-star Emilia Clarke will also play a leading role in Solo.
BBC
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