The Jersey hoard was found in a field by metal detectorists Richard Miles and Reg Meade in 2012.
The leather purse is about the size of an average palm and is described as an important find by senior conservator Neil Mahrer.
He said they have so far removed some 40,000 coins and this was the first bag they had come across.
Mr Mahrer said: "It is made of leather, it looks like it is either made of one sheet or bootlace strands.
"It is about 8cm across and 4cm deep. It is joined at one side to two rings that could have been for fixing it to a belt."
The team from Jersey Heritage have been working for the past 18 months on removing the coins one at a time. There are an estimated 70,000 coins in the hoard.
Mr Mahrer said: "The really important thing that might change our ideas is when we get to look at the coins themselves as we don't know the significance of the purse to the people who buried the hoard."
He said they would be removing all the coins surrounding the purse so they could then remove it with the earth in a single block.
The previous major find from the hoard was a selection of gold jewellery.
Mr Mahrer said: "Since removing the gold torques we thought we were in a coin-rich areas but seeing this it looks like we're in a rich seem of surprises again.
"Finding a purse where the whole thing is intact with the coins inside is pretty rare."
A rare purse has been found in the middle of one of the world's largest hoards of celtic coins.