Bitcoin's value has plummeted by 13% after hackers stole $65m (£49m) of the digital currency.
Hong Kong-based exchange Bitfines said it has halted trading, withdrawals and deposits after discovering the security breach.
Bitcoin exchances are a medium between traders, allowing them to buy and sell bitcoins while hoping to capitalise on fluctuations in its value.
Hackers managed to steal 119,756 bitcoin, which are worth around $543 (£408) each.
In a blog post Bitfinex said: "Today we discovered a security breach that requires us to halt all trading on Bitfinex, as well as halt all digital token deposits to and withdrawals from Bitfinex.
"We are deeply concerned about this issue and we are committing every resource to try to resolve it.
"We ask for the community's patience as we unravel the causes and consequences of this breach."
More than $1.5bn has been wiped off bitcoin's market capitalisation this week, reminiscent of the reaction after Mt Gox was hacked in February 2014.
Tokyo-based Mt Gox was the largest bitcoin exchange in the world at the time, and its compromise led to a 30% plunge in bitcoin prices that month.
One of the top comments on Reddit's bitcoin forum said: "Not sure we'll recover from this, since this seems to keep happening and is extremely bad press for bitcoin.
"At some point people might stop believing this thing has any future.
"I'm certainly having doubts now after more than five years. Perhaps people just aren't ready for this amount of freedom."