Bitcoins can be used for online money transfers and trades, and the currency uses cryptography to protect it.
But Bitfloor’s founder, Roman Shtylman, said he had kept unencrypted “keys”, which the thief accessed and used to take the money.
Bitfloor’s future is now in doubt.
Mr Shtylman said his New York-based service was the biggest of its kind in the US and the fourth largest in the world.
Unlike other currencies, Bitcoins are not issued by a central bank or other centralised authority. Instead they are created in a process called “mining”, in which coins are issued to a user when they solve a complicated mathematical problem using their computer.
More details: www.bbc.co.uk