The 2005 Nobel peace prize has been awarded jointly to the United Nations' International Atomic Energy Agency and its director, Mohamed ElBaradei.
The citation says the IAEA's director is a "fearless advocate" of curbing nuclear arms and the importance of his agency's work "incalculable".
The nuclear watchdog's work to promote safe nuclear energy was also commended.
Mr ElBaradei said the award recognised that the spread of nuclear weapons was the world's worst security threat.
Speaking at IAEA headquarters in Vienna, the Egyptian-born diplomat said he had learnt of the decision at home from live TV.
He began a third term at the IAEA last month after the US withdrew complaints that he was being "soft" on Iran.
Congratulating the prize-winners, the White House said the award underlined the importance of stopping nuclear weapons proliferation.
Ole Danboly Mjopes, head of the prize committee, stressed the award was not intended as a "kick in the shin" of any nation or leader but as a boost to disarmament.
'Policeman and salesman'
IAEA inspectors have had to deal with major crises in Iraq, North Korea and Iran in recent years as well as investigating the nuclear black market led by Pakistani scientist AQ Khan.
The BBC News website's Paul Reynolds notes that Mr ElBaradei, 63, makes his impact with expert analysis while remaining a fair judge.
But critics accuse the IAEA of failing to prove conclusively whether or not Iran's nuclear ambitions are peaceful, among other things.
And opponents of nuclear civilian energy condemn the agency's parallel commitment to civilian atomic power.
In the words of Greenpeace International, Mr ElBaradei is both "nuclear policeman and nuclear salesman".
This year's contest saw 199 names put forward - a record number.
21ST CENTURY PRIZEWINNERS
2005: Mohamed ElBaradei and the IAEA
2004: Wangari Maathai, Kenyan environmental campaigner
2003: Shirin Ebadi, Iranian human rights lawyer
2002: Jimmy Carter, former US president
2001: Kofi Annan, UN secretary general
2000: Kim Dae-jung, former South Korean president
Hidankyo, a Japanese association representing survivors of the 1945 atomic bomb attacks, said the decision in Oslo was "very disappointing".
The IAEA, said Hidankyo secretary general Terumi Tanaka, was "not an organisation that has conducted peace activities".
The annual international peace prize, worth $1.29m (£725,000), has been awarded since 1901, after its establishment by Swedish industrialist and inventor Alfred Nobel.
In keeping with tradition, it will be awarded in the Norwegian capital while the other Nobel prizes - for excellence in physics, chemistry, medicine and literature - are awarded in Stockholm.