The 220mm diameter 15kg coin was commissioned by a private UK collector for an undisclosed sum.
It is the largest coin the Mint says it has ever produced.
The £15,000 denomination coin was designed by coinage artist John Bergdahl and took nearly 400 hours to make.
The scale and diameter of the piece has allowed us to push the boundaries of minting to produce an exceptional level of relief and detail
Paul Morgan, Royal Mint
The reverse side depicts a crowned EIIR cypher surrounded by roses, daffodils, thistles and shamrocks, representing the United Kingdom.
On the obverse side, a commemorative design depicting the Queen on horseback is engraved on the precious metal.
Clare Maclennan, divisional director of commemorative coin at the Royal Mint, said: “This special edition coin made of 15 kilograms of fine gold is the masterpiece in the Platinum Jubilee collection, combining the finest craftsmanship and innovation rooted in our heritage as the original maker of coins for the monarchs and in celebration of Her Royal Majesty’s momentous 70 years on the throne.
“The largest coin ever made by the Royal Mint, it is a unique piece of art that will endure as a legacy of the occasion for generations to come.
“The one-off piece, commissioned by a private collector, represents our growth as the home of precious metals in the UK, making metals precious to our customers across all our collections.”
The Mint also issued a comment from the private UK collector, who said: “As a long-standing customer of the Royal Mint, I have invested in unique and interesting coins marking moments throughout the Queen’s reign that will remain in my family for generations.”
Paul Morgan, technical manager at the Royal Mint, said: “The scale and diameter of the piece has allowed us to push the boundaries of minting to produce an exceptional level of relief and detail.”
The Royal Mint has launched one of the largest ever commemorative collections for the Queen’s Platinum Jubilee.