Bidding began at $4.5 million and it took just two minutes to sell the stamp to an anonymous bidder on the phone, although the auction house Sotheby's had valued the tiny specimen of British colonial memorabilia at $10-20 million.
"With the premium the stamp has just sold for approximately $9.5 million, which means it has set a new world record price for a stamp," David Redden, Sotheby's director of special projects, told a packed room in New York.
Sotheby's said the previous auction record for a single stamp was $2.2 million, set by the Treskilling Yellow in 1996.
Made in 1856 in Guyana and measuring just one by 1.25 inches (2.54 by 3.18 centimeters), the one-cent magenta is octagonal, printed in black ink and bears the initials of the postmaster.
Redden had described the stamp as having "extraordinary fame and charisma" and being in remarkable condition given it is more than 150 years old.
Last bought by convicted murderer and American multi-millionaire John du Pont in 1980, it was last seen in public in 1986, before going on display at Sotheby's in the build-up to Tuesday's sale.
Hinged to a paper backing, since 1922 it has already broken three times the record price of a single stamp sold at auction.
The auction house says the stamp is the only surviving example of a one-cent magenta, so rare that it is missing even from the British royal family's philatelic collection.
- 'Shining star' -
It is "the largest shining star in the very distance in the great universe of collecting," Redden told AFP in February.
Colonial Guyana depended on supplies of stamps from England, but when a shipment was delayed in 1856, the postmaster commissioned a contingency supply.
The printers of the local Royal Gazette newspaper quickly ran off one-cent and four-cent magentas, and a four-cent blue.
The only surviving example of the one-cent was rediscovered in 1873 by Vernon Vaughan, a 12-year-old Scottish boy living with his family in British Guyana.
He found it among some family papers and added the stamp to his album. Vaughan then sold it to another collector for a few shillings and the stamp made its way to Britain in 1878.
It was bought by French Count Philippe la Renotiere von Ferrary, perhaps the greatest stamp collector in history, and later donated to a museum in Berlin.
After World War I, France seized his collection as part of war reparations due from Germany and sold the stamp in 1922 at auction to Arthur Hind, a textile magnate from New York.
Hind paid a then-record $35,000 for the stamp. It sold for a second record of $280,000 in 1970 and was bought in 1980 by the late du Pont for a third record of $935,000.
In a case that shocked the United States, du Pont shot dead Dave Schultz, an Olympic gold medal freestyle wrestler, at his estate in Pennsylvania in 1996 and died in prison in 2010.
The stamp is being sold by his estate. It has been on show at Sotheby's in London, Hong Kong and New York.
Current day Guyana, which won independence from Britain in 1966, is a small but poor nation of about 700,000 people.
$4 million is the auction record for any philatelic item, set in 1993 by The Bordeaux Cover, which includes both of the "Post Office" stamps issued by British colonial Mauritius.