The award seemingly came out of nowhere. But press secretary Robert Gibbs woke up President Barack Obama at 6 a.m. to tell him the news, the New York Times reports.
The last sitting American president to win the award was Woodrow Wilson in 1919. President Jimmy Carter won 20 years after he had left office.
The chairman of the committee making the award told the New York Times that the president already accomplished enough during only nine months of his presidency to deserve the award.
Thorbjorn Jagland, the chairman of the Norwegian Nobel Committee and a former prime minister of Norway, said the president had already contributed enough to world diplomacy and international understanding to earn the award.
Read more about the award from the Washington Post, including a long list of reactions to the announcement.