The coin, 36 millimetres in diameter and about 35 grams in weight, known as the “Português”, had been valued at between £20,000 and £30,000 pounds by London auctioneer Noonans.
According to the auctioneer, the lot attracted interest from bidders in the US and Canada, but was purchased by a buyer in Japan.
Despite the good condition, experts pointed out as a problem a mark that is thought to have been made by punching to certify that it was made of pure gold and whose cavity affects both sides of the coin.
First coined by King Manuel I, known as O Venturoso due to the impetus he gave to maritime explorations, the “Português” continued to be issued for 40 years, between 1498 and 1538, during the reign of João III.
The coin was produced in Lisbon with gold recovered by Vasco de Gama on his travels to Africa and India.
This specimen was discovered by accident in July by Mick Edwards, a 62-year-old civil servant with a metal detector he uses to look for buried objects.
“I was stunned and sat looking at the coin unable to breathe. I could see the cross on the coin and thought it was probably Spanish, but later I found out it was Portuguese", he said, quoted in a statement from the auctioneer.
The discovery took place before breakfast, at 06:00, in Etchilhampton, about 150 kilometres west of London, near the city of Bath, where Edwards was celebrating his 35th wedding anniversary.
According to the auctioneer, the land belonged to the Ernle family between 1489 and 1928, an aristocratic family in which John Ernle (1620-1697) stands out, who was a deputy and finance minister.
The family's status was reflected by the fact that Queen Anne visited the family home, Whetham House, 16 kilometres from where the coin was found in 1703.