Archaeologists are excited about the discovery because they say it sheds new light on the turbulent time, when Roman Britain suffered barbarian invasions, economic crises and civil wars.
Head of Portable Antiquities and Treasure at the British Museum, Roger Bland, said the hoard has "a huge amount to tell about the coinage and history of the period."
Dave Crisp, who found the hoard, said he hit the jackpot after his detector gave "a funny signal."
"I put my hand in, pulled out a bit of clay and there was a little Radial, a little bronze Roman coin. Very, very small, about the size of my fingernail," he told BBC radio.
Stephen Minnitt, Somerset County Council's head of museums said in a statement the find was of "great national importance."
Some of the coins are stamped with the image of Marcus Aurelius Carausius a military commander who seized power in the late third century and proclaimed himself emperor of Britain and northern Gaul.
Carausius who ruled from AD 286 to AD 293 was the first emperor to strike coins in Britain.
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