The piece sold for more than five times its estimated value
A Chinese bidder bought an 18th Century jade imperial seal for 1.68m euros (£1.5m) at a Paris auction house, despite protests by Chinese officials.
Beijing said the Qing Dynasty relic was looted by British and French troops from its Summer Palace in 1860.
The Chinese bidder refused to give his name, but said he was acting on behalf of an art collector in France.
The piece sold for more than five times its estimated value after a tense bidding race with another Asian buyer.
The authorities that manage Beijing's imperial Summer Palace said that all such relics should be returned to China.
They say the seal was originally taken by British and French troops from the palace in October 1860, towards the end of the Second Opium War.
China says the bronzes were taken abroad illegally 150 years ago
A statement said: "We once again express strong indignation at this sort of repeated action that hurts the Chinese people's feelings, harms their cultural interests, and violates international pacts."
The auction house said the seal, mounted with two carved dragons, came from the personal collection of a descendant of a French general who commanded some of the invading troops.
The sale comes two months after the contested sale of two bronze animal heads also said to have been looted from the Summer Palace.
A Chinese collector bought the heads of a rabbit and a rat for 15m euros ($19m; £13m) each, which were part of the collection of late French fashion designer Yves Saint Laurent.
The buyer then refused to pay, calling his decision an "act of patriotism".
The Chinese government, which had tried to stop the sale, denied any involvement. Christie's said the sale was legal, a position backed by a French court.