Scotland Yard, headquarters of the London police, said the headdress, depicting an image of a sea god, was recovered from a lawyers' office in central London and is considered to be of "phenomenal importance" to Peruvian cultural heritage.
Walter Alva, director of the Royal Tombs of Sipan Museum in Peru, said on Thursday: "We are speaking about an archaeological object of the utmost historical and aesthetical importance, one of the most important ornaments of the ancient Peruvian cultures."
The artefact, made of gold, symbolises a mythical octopus with eight tentacles and a stylised human head displaying cat-like features.
It is thought to date from about 700 AD and to be an example of ancient Peruvian Mochica civilisation art.
"No ornament of similar quality can be found in any Peruvian museum and it is inconceivable that such an important part of national treasure is out of our country," Alva said in a statement.
British detectives believe the headdress was taken from the La Mina archaeological site in the Jequetepeque valley in north Peru in 1988 when a tomb was looted and its contents put on the black market.
No arrests have been made in connection with the seizure and no further details of the operation were immediately available.