The unnamed 25-year-old told three clients of the Osaka-based bank last year that he could generate profits if they entrusted their cash to him.
The banker, who worked at the bank's Ikebukuro branch in Tokyo, collected a total of USD 1.5 million from them without informing his superiors, said a spokesman with the bank.
"But most of the money appeared to be lost as he allegedly used the funds in foreign exchange and other trading," the spokesman said.
The following day the man killed himself, the spokesman said.
The bank prohibits its employees from collecting funds for investment without permission.
The case was "extremely regrettable," the spokesman said, adding that the bank has reported it to the Financial Service Agency.