The move was approved by Fisher's family and her daughter Billie Lourd.
Lourd will also appear in Episode IX again as Lieutannant Connix, after she took on the role in Episode VII.
"We desperately loved Carrie Fisher," Abrams said.
"Finding a truly satisfying conclusion to the Skywalker saga without her eluded us.
"We were never going to recast, or use a CG character.
"With the support and blessing from her daughter, Billie, we have found a way to honour Carrie's legacy and role as Leia in Episode IX by using unseen footage we shot together in Episode VII."
Another stalwart actor to return after speculation is Billy Dee Williams, who will reprise his role as Lando Calrissian.
It will be his first Star Wars film in 35 years, when he last filmed Star Wars Episode VI: Return of the Jedi. Williams first appeared in Star Wars: Episode V — The Empire Strikes Back as an old friend of Han Solo (Harrison Ford).
While Williams has not been in any Star Wars movies for more than three decades, he has been voicing the animated character of Lando Calrissian in videos games and cartoons.
Many fans will be pleased with the addition of Williams, after they flooded fan sites with calls for him to return.
Many were disappointed he did not appear in Star Wars Episode VII: The Force Awakens or Star Wars Episode VIII: The Last Jedi.
Mark Hamill, who died in Episode VIII: The Last Jedi, will also return to Episode IX, alongside Anthony Daniels, who has played the famous droid, C-3PO.
Returning cast members include Daisy Ridley who plays Rey, Adam Driver who plays the two-sided character of Kylo Ren, John Boyega as Finn, Oscar Isaac as Poe Dameron, Lupita Nyong'o as Maz Kanata, Domhnall Gleeson as General Hux, Kelly Marie Tran as Rose, and Joonas Suotamo as Chewbacca.
Composer John Williams, who has scored every chapter in the Star Wars saga since 1977's A New Hope, will return to Episode IX.
Star Wars: Episode IX will be produced by Kathleen Kennedy, J.J. Abrams, and Michelle Rejwan, and executive produced by Callum Greene and Jason McGatlin.
The film is set to be released in December 2019.