"We think it is in our national security interest to be as transparent as we can be about the nuclear program of the United States," Secretary of State Hillary Rodham Clinton told reporters at the United Nations, where she addressed a conference on containing the spread of atomic weapons.
The United States previously has regarded such details as top secret.
The figure includes both "strategic," or long-range weapons, and those intended for use at shorter range.
The Pentagon said the stockpile of 5,113 as of September 2009 represents a 75 per cent reduction since 1989.
A rough count of deployed and reserve warheads has been known for years, so the Pentagon figures do not tell nuclear experts much they did not already know.
Hans Kristensen, director of Nuclear Information Project, Federation of American Scientists in Washington, said his organization already had put the number at around 5,100 by reviewing budget estimates and other documents.
The import of the announcement is the precedent it sets, Kristensen said.
"The important part is that the U.S. is no longer going to keep other countries in the dark," he said.
Clinton said the disclosure of numbers the general public has never seen "builds confidence" that the Obama administration is serious about stopping the spread of atomic weapons and reducing their numbers.
The administration is not revealing everything.
The Pentagon figure released Monday includes deployed weapons, which are those more or less ready to launch, and reserve weapons. It does not include thousands of warheads that have been disabled or all but dismantled. Those weapons could, in theory, be reconstituted, or their nuclear material repurposed.
Estimates of the total U.S. arsenal range from slightly more than 8,000 to above 9,000, but the Pentagon will not give a precise number.
Whether to reveal the full total, including those thousands of nearly dead warheads, was debated within the Obama administration. Keeping those weapons out of the figure released Monday represented a partial concession to intelligence agency officials and others who argued national security could be harmed by laying the entire nuclear arsenal bare.
A senior defence official, speaking on condition of anonymity because the overall total remains classified, did not dispute the rough estimates developed by independent analysts.
Exposure of once-classified totals for U.S. deployed and reserve nuclear weapons is intended to nudge nations such as China, which has revealed little about its nuclear stockpile.
"You can't get anywhere toward disarmament unless you're going to be transparent about how many weapons you have," said Sharon Squassoni, a nuclear policy analyst at the Center for Strategic and International Studies.
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