The book, called The Pike and written by Lucy Hughes-Hallet, won the accolade, along with a £20,000 (€23,800) prize, on Monday night. The biography fended off competition from finalists including Charles Moore's biography about the UK's late former prime minister, Margaret Thatcher.
It tells the story of D'Annunzio's life as a writer and revolutionary, charting his rise to national hero.
The controversial poet, who was also known for his high-profile affairs with several women, made an impact in politics as well as through his writing, with the two often being intertwined. At one point, he was seen as a rival to dictator Benito Mussolini.
"Writing about him meant I was writing about the entire cultural ambience in which he lived," Hughes-Hallet recently told BBC Radio 4.
"He was a decadent and he was a great enjoyer of the good things of life - flowers and poetry and sex; he was an enthusiastic and surprisingly successful seducer."
Martin Rees, chair of the judges and Royal Astronomer, said Hughes-Hallett's narrative “transcends the conventions of biography.”
He added that readers “will be transfixed by her vivid portrayal of D'Annunzio – how this repellent egotist quickly gained literary celebrity and how his oratory influenced Italy’s involvement in the First World War and the rise of Mussolini. The book shows how fascism rose partly as a perversion of nationalism - a trend still sadly relevant in today's world.”