The Japanese Bankers Association said on Tuesday glitches at its payment clearing network system disrupted more than 1.4 million domestic fund transfers.
It's the first major system failure that the banking group has experienced.
The failure at the system's relaying computers blocked transfers to and from 11 banks including MUFG Bank, Resona Bank, Yamaguchi Bank and JPMorgan Chase Bank.
A total of 1.4 million transfers from the 11 banks were affected on Tuesday, the operator of the system said.
Of those, 1 million were handled by alternative methods, while 400,000 remained unprocessed as of Tuesday evening.
An unspecified number of transfers addressed to those 11 banks from other banks were also blocked.
It's not clear when the system can be fully restored, the operator said, adding that it is investigating the cause jointly with its system vendor NTT Data Group.