A Storm of Swords, the third book from George RR Martin's Song of Ice and Fire novel series, will be spread over two seasons of the HBO drama.
The teenage Reed siblings, Catelyn Stark's uncle and Stannis Baratheon's wife and daughter will be among the new characters introduced in the third season, according to Entertainment Weekly.
> Read Digital Spy's review of the latest Game of Thrones episode 'Blackwater'
The list of new characters is as follows:
Mance Rayder: Previously mentioned in the series, a former member of the Night's Watch who became known as the 'King Beyond the Wall', and the leader of the Wildlings.
Daario Naharis: A seductive warrior.
Jojen Reed and Meera Reed: A teenage brother and sister with "special insights".
Edmure Tully: A young member of the Tully family.
Ser Brynden Tully (The Blackfish): Catelyn Stark's uncle.
Lady Selyse Florent: Stannis Baratheon's wife.
Shireen: Stannis's daughter.
Olenna Redwyne (The Queen of Thorns): Margaery Tyrell's grandmother.
Beric Dondarrion: A skilled knight, the leader of the outlaw group Brotherhood Without Banners.
Thoros of Myr: A red priest.
Tormund Giantsbane: A Wildling raider.
Showrunner David Benioff said of the new characters: "It's important to point out that we have the largest cast on television right now. We introduced dozens of new characters in season two.
"If you hurl 300 characters at an audience, the story collapses under the weight of too many faces, too many names, and too many subplots.
"We need to be just as mindful of the audience members who have never read the books as we are of the readers; the series will fail if we only appeal to those who already know the characters.
"So we try to be parsimonious about how many new roles we introduce to the story and when we introduce them."
On whether there may be more characters to those already listed, he added: "And a few others... holy hell, that's a lot of new faces."
The season two extended finale of Game of Thrones will air on Sunday, June 3 on HBO in the US, and on Monday, June 4 on Sky Atlantic in the UK.