Federal agents said the man in the video above went into Old National Bank on Dixie Highway in Chicago Heights on Friday around 5 p.m.
He allegedly showed the teller a silver handgun and demanded money before taking off on foot.
The FBI said the same man robbed Fifth Third Bank in Oak Forest within an hour of the first robbery.
The suspect is described as between 30 and 40 years old with a medium build. He was last seen wearing a grey or beige t-shirt, light-colored jeans with ripped knees, a black cross-body bag, black sneakers and a dark-colored baseball cap.
Anyone with information is asked to call police.