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."

Warnock, 68, has a meeting with chairman Mehmet Dalman next week to discuss his future.
But having steered Cardiff away from the Championship relegation zone during his five-month reign, Warnock says he feels "refreshed".
"I might even do a couple [of years] whatever the situation," he said.
After back-to-back wins on the road against Leeds and Derby, on Saturday Warnock will cross swords with Rotherham, who he saved from relegation last season during a 16-game spell from February until June.
The Millers were keen to keep Warnock last summer and he admits he was close to staying with them.
"Rotherham actually gave me that zest back," he said.
"If I am honest I might have let my heart rule my head at times. If we had agreed terms straight away I think I might have stayed."
Now Warnock feels similarly revitalised having taken over at Cardiff in October.
"It's given me the belief in what I do really. I have been a manager 37 years, that is a long time - more than a lot of my lads have ever been born," he said.
"To still have that enthusiasm and that desire to want to put one over people or managers or clubs. That is what I thrive on.
"The biggest thing is for Cardiff fans to go home having seen a performance or effort of their team and to be talking about the club again in the good sense rather than a negative sense."
Warnock said Cardiff chairman Dalman would provide him with financial details on what funds could be available this summer at a meeting in the next few days.
"We need to know what we can do to be able to bring the three or four players in because we don't have that many players out of contract really, so it is not as straightforward to thin my squad or to do things," Warnock added.
"So its going to be difficult in the next few months to put things in place.
"It is nice to be wanted. The biggest thing coming here was the reaction of the Cardiff fans to me taking over, it was fantastic.
"It would have to be something extreme I have not thought about to take me away from that."
A rare purse has been found in the middle of one of the world's largest hoards of celtic coins.
[NEXT_CONCEPT]
Manager Neil Warnock has delivered the strongest indication yet he is keen to stay at Cardiff City and mount a promotion challenge next season.