Two metal detector enthusiasts found the hoard, dated to the 1st century B.C. and described by experts as "extremely exciting and very significant."
The hoard weighed about three-quarters of a ton and could contain about 50,000 coins.
Each individual coin is worth between $150 and $300, Philip de Jersey, a former Celtic coin expert at Oxford University, told the BBC.
Metal detector hobbyists Reg Mead and Richard Miles said they had been searching for more than 30 years after hearing rumors a farmer had discovered silver coins while working on his land in the east of Jersey.
A large mound of clay containing the coins has been taken to a safe location to be studied, researchers said.
Authorities have not revealed the exact location of the hoard.
"Sites like these do need protection because there is speculation there might even be more," Environment Minister Deputy Rob Duhamel said.
The hoard of coins is the first to be found on Jersey for more than 60 years, officials said.