It is one of only 11 known surviving copies of the US charter.
An extremely rare original copy of the US Constitution has sold for $43.2 million (€38 million) in New York on Thursday, a world record for a historical document at an auction, according to Sotheby's.
It is one of only 11 known surviving copies of the US charter, which was signed in Philadelphia's Independence Hall by founding fathers, George Washington, Benjamin Franklin and James Madison in 1787.
The document was originally estimated between $15 million and $20 million.
"It's incredibly rare," said Selby Kiffer, a manuscripts and ancient books expert at Sotheby's, noting that it was probably printed on the eve of the signing.
"This was probably printed in an edition of 500 copies. Now only eleven are known to survive and this is the only one in private hands," he added.
Kiffer is not worried if the famous document, with the immortal opening words "We the People", goes abroad, even though he believes it will stay in the United States.
"It's the longest-serving charter of government in the world. I think it's already been a model for other constitutions around the world," Kiffer explained.
"But I think when it comes down to it, no one values American history more highly than Americans and I would be surprised if the new owner is not an American," he went on.
The document is in excellent condition and belonged to American collector Dorothy Tapper Goldman.
Sotheby's is exhibiting until Sunday a private collection of American constitutional documents from the revolutionary period from 1776 to 1789.