The US Defence Department announced that the US has 5,113 nuclear warheads, disclosing the size of the atomic arsenal for the first time.
The decision to release the previously secret figure comes as President Barack Obama tries to encourage other countries to be more transparent about their nuclear activities.
The Pentagon said the current figure is down from 22,217 warheads in 1989, near the end of the Cold War.
US Secretary of State Hillary Clinton told a UN nuclear conference in New York that the Pentagon would be releasing the number to "improve the transparency of our nuclear arsenal".
"For those who doubt that the US will do its part on disarmament, this is our record, these are our commitments. And they send a clear, unmistakable signal," she said.
Clinton was in New York to attend the UN review conference of the 40-year-old nuclear Non-Proliferation Treaty amid concerns that the pact was being weakened by Iran's and North Korea's secretive nuclear activities.
The figures include warheads that can be deployed on missiles, submarines or bombers.
Nuclear experts and non-governmental organizations have long produced close estimates of the US stockpile. The number of US warheads peaked in 1967 at 31,255.