Christie's auction house sold the 28.5-centimetre (11-inch) relic for £4,746,250 (around AU$8.5 million) at one of its most controversial auctions in years.
No information about the buyer was disclosed.
The famous pharaoh's finely-chiselled face -- its calm eyes and puffed lips emoting a sense of eternal peace -- came from the private Resandro Collection of ancient art that Christie's last auctioned off 2016 for £3 million.
But angry Egyptian officials wanted Thursday's sale halted and the treasure returned.
Christie's decision "contradicts international agreements and conventions," Egypt's foreign ministry said on Wednesday.
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