Ali Smith has won the Baileys Women's Prize for Fiction for her time-shifting novel How to Be Both.
The judges hailed the Scottish writer's work as "tender, brilliant and witty".
A visibly shocked Smith was presented with the £30,000 prize at a ceremony at London's Royal Festival Hall on Wednesday night.
"It's unbelievable, it's glorious and it feels like an astonishing fluke," she told the BBC after her win.
"It was such a strong, versatile shortlist, and I can't believe this has happened."
Smith's book beat novels by writers including Rachel Cusk, Sarah Waters and Anne Tyler.
The writer said she was set to start work on a new novel in July - based on an idea she had 20 years ago - which she said would be a "whole new challenge".
A novel of two halves, How to be Both's dual narrative focuses on the lives of a grieving teenage girl in the present day and a 15th Century Renaissance artist.
The book is published in two versions - with the same cover - but with the two halves switched. It doesn't matter in which order they are read.
"Ancient and modern meet and speak to each other in this tender, brilliant and witty novel of grief, love, sexuality and shape-shifting identity," said chair of judges Shami Chakrabarti, head of civil rights group Liberty.
The other judges were Laura Bates, founder of the Everyday Sexism Project, columnist and broadcaster Grace Dent, writer Helen Dunmore and Channel 4 News presenter Cathy Newman.
The idea for the novel took root when Smith saw a picture of a fresco in a 2013 edition of art magazine Frieze.
"I took a mouthful of coffee and opened it at a full-page reproduction of a painting so beautiful that it did something to my breathing and I nearly choked," she wrote in The Observer.
The Bookseller has reported that publisher Penguin Random House is printing 40,000 more paperback copies of the book following her win.
Simon Prosser, Smith's publisher at Hamish Hamilton, said: "We are so, so happy".
How to be Both has already made its mark on the literary scene, having won the 2014 Costa Novel Award and the 2014 Goldsmiths Prize. It was also shortlisted for the 2014 Man Booker Prize and the 2015 Folio Prize.
Born in Inverness in August 1962, Smith lives in Cambridge. As well as four short story collections, her novels include Hotel World and The Accidental, which won the Whitbread Novel Award.
Smith was made a CBE for services to literature in the 2014 New Year's Honours List.
Formerly known as the Orange Prize for Fiction, the Baileys Prize is open to any woman writing in English.
Last year's prize went to A Girl is a Half-Formed Thing by Ireland's Eimear McBride.
In addition to the Baileys Prize announcement, Claire Rilett was named as the winner of the First Chapter competition for unpublished writers.
Baileys Women's Prize for Fiction 2015 shortlist
- Outline by Rachel Cusk
- The Bees by Laline Paull
- A God in Every Stone by Kamila Shamsie
- How to be Both by Ali Smith
- A Spool of Blue Thread by Anne Tyler
- The Paying Guests by Sarah Waters