Paul Flack, who had recently started metal detecting, discovered the find of more than 600 copper alloy coins in a field last October.
Experts say the find was of the usurper emperors Carausius (287-293 AD) and Allectus (293-296 AD).
At an inquest, the find was declared treasure. The finder will be compensated for its value.
John Newman, of Suffolk County Council's Archaeological Service, said: "This appears to be the largest hoard of legitimately-minted coins of the two usurpers from Britain to date.
"The coins are made up of 258 of Carausius, and 347 of Allectus, minted at London and possibly Southampton or Colchester, which was the first time official mints were set up in Roman Britain."
The coins are currently being kept at the British Museum, where they will be cleaned for valuation by the Treasure Valuation Committee.
Mildenhall Museum hopes to buy the treasure.