Unlike old currencies bitcoin is digital, traded computer-to-computer, and often used to pay for goods and services on the internet.
But it has no central authority or government-based backing, and the alleged theft - one of the largest in the currency's four-year history - raises questions about digital money and if it can ever be regulated.
A bitcoin user who calls himself TradeFortress says more than 4,100 bitcoins worth about $1.1 million have been robbed from his website.