The Mermaid of Black Conch, a dark love story about a fisherman and a mermaid torn from the sea, has won the Costa Book of the Year award.
Trinidadian-born British writer Monique Roffey beat four other contenders with her sixth novel to scoop the £30,000 prize.
Judges said the book was "utterly original... and feels like a classic in the making".
A "delighted" Roffey said her win was a vote for Caribbean literature.
"A huge thank you to the judges for exposing my book to a wide readership. I'll be pinching myself for weeks to come," she added.
Based on a Taino legend of a beautiful woman transformed into a mermaid, the story is set in the Caribbean village of St Constance.
David, a fisherman, unexpectedly attracts the attention of Aycayia, a mermaid who is drawn to his singing. When she is captured from the sea during an annual fishing competition, he does all he can to save her, with dramatic consequences.
Professor Suzannah Lipscomb, chair of judges, said: "The Mermaid of Black Conch is an extraordinary, beautifully written, captivating, visceral book - full of mythic energy and unforgettable characters, including some tremendously transgressive women."
Analysis from Rebecca Jones, BBC Arts correspondent
The Costa Book Awards have a reputation for picking popular reads: books you would recommend to a friend. And I would definitely recommend The Mermaid of Black Conch.
At first, the novel might sound a bit odd. Set on a Caribbean island in the 1970s, it is a bittersweet love story between a beautiful young woman cursed to live as a mermaid and a fisherman.
Based on a legend passed down by the indigenous people of the Caribbean, the Taino, there are touches of magic and snippets of poetry. The book was also shortlisted for the Goldsmiths Prize last year, which rewards fiction that breaks the mould or extends the possibilities of the novel.
But while it is unusual it is also a joy to read, brimming with memorable characters and vivid descriptions.
We see the mermaid's "hair flying like a nest of cables" while we are told "sea moss trailed from her shoulders like slithers of beard" and "barnacles speckled the swell of her hips."
For me, this was a hugely entertaining and thought-provoking novel and a worthy winner.
Roffey, a senior lecturer in creative writing at Manchester Metropolitan University, secured her publishing deal through Peepal Tree Press, an independent publisher supporting Caribbean writers.
She then crowd-funded her publicity campaign with the support of fellow authors.
Roffey's entry was also named Costa's Novel of the Year earlier this month, alongside winners from four other categories:
- First Novel Award - Love After Love by Ingrid Persaud
- Children's Book Award - Voyage of the Sparrowhawk by Natasha Farrant
- Poetry Award - The Historians by Eavan Boland
- Biography Award - The Louder I Will Sing by Lee Lawrence
The Mermaid of Black Conch is the thirteenth novel to take the overall prize. Days Without End by Sebastian Barry was the last novel to be named Costa Book of the Year in 2016.
Tuesday's virtual ceremony also saw London-based writer Tessa Sheridan receive the 2020 Costa Short Story Award.
Sheridan won the public vote and £3,500 for her story, The Person Who Serves, Serves Again.
The Costa Book Awards, formerly the Whitbread Book Awards, were established in 1971 to encourage, promote and celebrate the best contemporary British writing.
It is open to UK and Irish authors.
Seamus Heaney, Ted Hughes and Sebastian Barry are among the authors to have won the book of the year award more than once.