RBI said it had received a complaint regarding "huge cash withdrawals in certain accounts" maintained with Union Bank of India.
The penalty was imposed on July 26.
"This action is based on deficiencies in regulatory compliance and is not intended to pronounce upon the validity of any transaction or agreement entered into by the bank with its customers," the RBI added.
The central bank said it had examined the documents regarding the complaint and notice was issued to the state- owned lender.
The RBI said that after considering the bank's reply, oral submissions, additional information and documents, it concluded that the charge was substantiated and warranted imposition of monetary penalty.