Since the first planned prequel series starring Naomi Watts (set 10,000 years before GOT) was not picked up by HBO, House of the Dragon will be GOT's first spin-off — and the only one currently, officially greenlit.
News broke in March 2021 that HBO is reportedly making three more spin-off series, with The Sea Snake, formerly titled Nine Voyages, the furthest along in development (representatives for HBO declined to comment).
Another project reportedly in development at HBO is set about 1,000 years before the events of GOT and would follow Princess Nymeria, an ancestor of House Martell who founded Dorne.
Game of Thrones star Kit Harington is reportedly attached to reprise his role as Jon Snow in a spin-off series that is in very early stages.
HBO is also reportedly developing a series based on Martin's Tales of Dunk and Egg novellas about iconic knight Ser Duncan the Tall (Dunk) and his squire (Egg) — who is destined to become King Aegon V Targaryen.
Another prequel reportedly in development is set in Flea Bottom, the King's Landing slum featured in the original series
Since the first planned prequel series starring Naomi Watts (set 10,000 years before GOT) was not picked up by HBO, House of the Dragon will be GOT's first spin-off — and the only one currently, officially greenlit.