British novelist Bernardine Evaristo has become the fire black woman to win the prize since it was launched in 1969.
But she will share the £50,000 prize with Canadian author Margaret Atwood, the bookmakers’ favourite and a previous winner.
Atwood and Evaristo were two of four women on the shortlist for the prize this year, along with Edinburgh-based author Lucy Ellman, who was born in America, and Turkish novelist Elif Sharak.
Another previous winner, British-Indian writer Salman Rushdie, and Nigerian writer Chigozie Obioma made up the final six.
Speaking after the announcement at the awards ceremony in London, Atwood, at 79 the oldest winner of the prize, said: "Neither of us expected to win this.
"I'm very surprised. I'd have thought I would have been too elderly and I kind of don't need the attention. It would have been quite embarrassing for me, a good Canadian, as we don't do famous, if I have been alone here."
Evaristo, 60, added: "It's so incredible to share this with Margaret Atwood, who is such a legend. A lot of people say this, but I never thought it would happen to me. I will say this - I am the first black women to win this prize and I hope that honour doesn't last too long. I hope other people come forward now."
Atwood, who won in 2000 for The Blind Assassin, has become only the fourth author to win the prize twice, this year for The Testaments, a sequel to her 1985 novel The Handmaid’s Tale, which was shortlisted for the prize, but lost out to Kingsley Amis’s novel The Old Devils.
In a review of The Testaments – which is set 15 years after the end of Atwood’s original dystopian novel – for The Scotsman, critic Stuart Kelly described Atwood’s sequel as “fluent, imaginative and provocative”.
Atwood has said of the new book: “In many ways, The Testaments is an answer to all the questions readers have been asking me about The Handmaid’s Tale over the years.”
London-born Evaristo, 60, has won the prize with her eighth novel, Girl, Woman, Other, which follows the lives and struggles of 12 different characters, who are mostly women, black and British.
She has said of the book: “I was very frustrated that black British women weren’t visible in literature.”
Judging panel chair Peter Florence said: “In the room today we talked for five hours about books we love, two novels we cannot compromise on. They’re both phenomenal books that will delight readers and resonate for ages to come.”
Gaby Wood, literary director of the Booker Prize Foundation, said: “Over an agonising five hours, the judges discussed all of the much-loved books on their shortlist and found it impossible to single out one winner.
“They were not so much divided as unwilling to jettison any more when they finally got down to two, and asked if they might split the prize between them.
"On being told that it was definitively against the rules, the judges held a further discussion and chose to flout them.”
The judges decided to announce two winners, despite a rule change in 1993 which insisted only one author could win the prize.
The previous year the prize had been shared by Michael Ondaatje and Barry Unsworth for The English Patient and Sacred Hunger respectively.
The only other year the judges failed to select an outright winner was in 1974, when the prize was shared between Nadine Gordimer, for The Conservationist, and Stanley Middleton for Holiday.