American wins France's top literary prize for his novel about a fictional SS officer's memoirs
The prize jury said Littell's debut novel easily triumphed over a field of three other works from French authors with a 7-3 vote.
The book is already a sensation in France, where it sits atop the best-seller list with more than 250,000 copies sold.
Littell, the 39-year-old son of US journalist and spy novel writer Robert Littell, was not present to receive the honour.
His French editor said he remained at his home in Barcelona, Spain, and transmitted a message saying "he prefers to stay out of the limelight."
Littell "is very happy and he accepts this prize with pleasure," added Antoine Gallimard, of the Gallimard publishing house, stressing that no form of disrespect was intended by his absence.
"He has no time for publicity, partly out of shyness, but also because he believes literature is not an entertainment industry. What is important is the book itself," Gallimard said, adding that another 150,000 copies would now be printed.
Born in New York in 1967, Littell moved to France and spent his childhood there until the age of 18, when he returned to the United States for university.
After graduating, he spent 15 years travelling around the world, much of it working for humanitarian organisations.
Fully bilingual, he wrote his 900-page book in France, though admitted that he wrote up much of his methodical research in English.
"Les Bienveillantes" has already won a prestigious Academie Francaise prize given to first-time authors writing in French.
It is soon to be translated and published in Britain, the United States and other countries after auctions believed to have netted Littell more than a million dollars.
The novel tells the story of an unrepetant Nazi SS officer who recounts his extermination of Jews in World War II.
"What interested me was to understand what led people to become torturers," Littell has said in one of his rare interviews.
The premise, though, has earned criticism from some quarters, and a German historian, Peter Schottler, has called the book inaccurate and unconvincing.
The head of the Goncourt jury, Edmonde Charles-Roux, brushed those views aside, enthusiastically endorsing Littell's book after Monday's vote.
"You can't dismiss such a monument," he said.
"The vote was definitive, as it always is for the Goncourt. It was very sharp: some for, others against, but it was even so an impressive vote," Charles-Roux said.
A jury member, Jorge Semprun, said he was "stunned by this amazing book -- it's the literary event of this half-century."
Littell's triumph was the high point of an invasion of foreign writers who have invigorated France's often staid literary world by sweeping several top prizes.