Japanese cryptocurrency exchange DMM Bitcoin said on Friday that it had lost 4,502.9 bitcoin, worth about ¥48.2 billion ($308 million), in what the company called an "unauthorized leak."
It gave no details of how the incident occurred or whether an outside party had been involved, but said it was investigating and had restricted some services in response.
Cryptocurrency companies are frequent targets for hacks and cyberattacks, although losses of this scale are rare.
DMM Bitcoin said it would replace customers' lost bitcoin deposits with the help of other group companies.
The Financial Services Agency demanded that the company investigate the incident, NHK said.
Tom Robinson, chief scientist and co-founder of the crypto research firm Elliptic, said that if DMM's loss turned out to be theft, it would be the eighth largest crypto theft of all time, based on exchange rates at the time it occurred.
It would be the largest since the $477 million hack suffered by FTX in November 2022, he added.
Crypto news outlet CoinDesk said DMM's loss was the second largest the region had experienced, after $530 million worth of cryptocurrency was stolen from the Japanese crypto exchange Coincheck in 2018.
Around $1.7 billion was stolen from various cryptocurrency platforms last year, crypto research firm Chainalysis said.