"He issued a demand to hand him over a certain amount of money," police in the city of Belgorod said in a statement, without saying how much he had asked for.
"There is information that there are three people inside the bank," police said, adding that the building and nearby area had been cordoned off.
Regional police spokesman Yevgeny Kamalov told AFP that talks were under way to release the hostages.
A spokeswoman for the regional branch of the FSB security service identified the lender as the Zapadny Bank.
As of April 1, 2014, the lender was the 142th largest bank in Russia according to its assets, the Central Bank said.