The Roman and Celtic coins, which date from the 1st Century BC, were found by two metal detector enthusiasts.
Dr Philip de Jersey, a former Celtic coin expert at Oxford University, said the haul was "extremely exciting and very significant".
He said each individual coin was worth between £100 and £200.
The exact number of coins found has not been established, but archaeologists said the hoard weighed about three quarters of a tonne and could contain about 50,000 coins.
The exact location of the hoard has not been revealed by the authorities but Environment Minister, Deputy Rob Duhamel, said he would do everything he could to protect the site.
"Sites like these do need protection because there is speculation there might even be more," he said.
"It is a very exciting piece of news and perhaps harks back to our cultural heritage in terms of finance. It was found under a hedge so perhaps this is an early example of hedge fund trading."
It was found by Reg Mead and Richard Miles in a field in the east of Jersey.
They had been searching for more than 30 years after hearing rumours a farmer had discovered silver coins while working on his land.
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Mr Mead and Mr Miles worked with experts from Jersey Heritage to slowly unearth the treasure.
A large mound of clay containing the coins has now been taken to a safe location to be studied.
It is the first hoard of coins found in the island for more than 60 years.
Several hoards of Celtic coins have been found in Jersey before but the largest was in 1935 at La Marquanderie when more than 11,000 were discovered.
Dr de Jersey said it would take months for archaeologists to find out the full value of the haul.
He said: "It is extremely exciting and very significant. It will add a huge amount of new information, not just about the coins themselves but the people who were using them.
"Most archaeologists with an interest in coins spend their lives in libraries writing about coins and looking at pictures of coins.
"To actually go out and excavate one in a field, most of us never get that opportunity. It is a once in a lifetime opportunity."
The ownership of the coins is unclear. Mr Mead said he had asked the States of Jersey for clarification.
Deputy Duhamel said the owners of the site had indicated they would like to see the whole hoard on display at the Jersey Museum or the archive.
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