The 2009 Nobel Peace Prize was awarded to United States president Barack Obama (b. 1961) for his "extraordinary efforts to strengthen international diplomacy and cooperation between peoples". The Norwegian Nobel Committee announced the award on October 9, 2009, citing Obama's promotion of nuclear nonproliferation and a "new climate" in international relations fostered by Obama, especially in reaching out to the Muslim world.

The Nobel Committee's decision drew mixed reactions from US commentators and editorial writers across the political spectrum, as well as from the rest of the world.

Obama accepted the prize in Oslo on December 10, 2009. In a 36-minute speech, he discussed the tensions between war and peace and the idea of a "just war" saying, "perhaps the most profound issue surrounding my receipt of this prize is the fact that I am the commander-in-chief of the military of a nation in the midst of two wars."

Obama is the fourth president of the United States to have won the Nobel Peace Prize (after Theodore Roosevelt, Woodrow Wilson and Jimmy Carter, with Carter's honor happening after leaving office).

The winner is selected by the Nobel Committee from nominations submitted by committee members and others. Nominations for the 2009 Nobel Peace Prize closed just 11 days after Obama took office. There were 205 nominations for the 2009 award, which included Chinese and Afghan civil rights activists and African politicians. Colombian senator Piedad Córdoba, Afghanistan's Sima Samar, Chinese dissident Hu Jia and Prime Minister of Zimbabwe Morgan Tsvangirai had been speculated to be favorites for the award.

The five members of the Nobel Committee are appointed by the Norwegian Parliament to roughly reflect the party makeup of that body. The 2009 Committee comprised two members of the Norwegian Labour Party, one from the left-wing Socialist Left Party, one from the Conservative Party of Norway and one from the right-wing Progress Party. The chairman of the committee was Thorbjørn Jagland, former Norwegian Labour Party prime minister and Secretary General of the Council of Europe since September 29, 2009. The panel met six or seven times in 2009, beginning several weeks after the February 1 nomination deadline. The winner was chosen unanimously on October 5, but was initially opposed by the Socialist Left, Conservative and Progress Party members until strongly persuaded by Jagland.

Jagland said "We have not given the prize for what may happen in the future. We are awarding Obama for what he has done in the past year. And we are hoping this may contribute a little bit for what he is trying to do," noting that he hoped the award would assist Obama's foreign policy efforts. Jagland said the committee was influenced by a speech Obama gave about Islam in Cairo in June 2009, the president's efforts to prevent nuclear proliferation and climate change, and Obama's support for using established international bodies such as the United Nations to pursue foreign policy goals.

The New York Times reported that Jagland shrugged off the question of whether "the committee feared being labeled naïve for accepting a young politician's promises at face value", stating that "no one could deny that 'the international climate' had suddenly improved, and that Mr. Obama was the main reason... We want to embrace the message that he stands for."