The 840 handmade coins, called staters, were unearthed in a field near Wickham Market, Suffolk, in March last year.
After Michael Dark made the discovery with his metal detector, archaeologists found more coins, which are now at the British Museum in London.
Experts believe they were produced by predecessors of the Iceni queen Boudicca and date from 40BC to AD15.
It is the largest haul of coins to be discovered in 150 years and could be worth up to £500,000.
Rare find
Only a few hoards of similar Icenian coins, which depict a horse on one side, have ever been found.
The Iceni lived in Norfolk, much of Suffolk, and parts of Cambridgeshire during the century before the Roman conquest in 43AD.
Archaeologists think the coins were probably buried in the early years of the 1st Century, about 25 years before the Romans arrived.
Only the bottom half of a pot containing some of the coins had survived, the rest was smashed during cultivation of the field.
Mr Dark told the BBC: "I knew there was a lot there but did not realise it was as significant as it was."
He said he would like to see the hoard displayed at Ipswich Museum.
Suffolk coroner Dr Peter Dean declared the hoard treasure, as the coins were more than 300 years old.
During the treasure trove inquest, Dr Dean heard how most were produced in Suffolk and Norfolk, but why such a large hoard was buried is still unclear.
Source:news.bbc.co.uk