Danbury police said the woman hid shortly before 6 p.m., when the bank was closing. She came out when only a few employees and a cleaning crew were left inside, police said.
The woman then went up to a teller and "demanded all the money," police said. No weapon was displayed during the robbery, police said.
Police described the suspect as an African-American female, about 5 feet, 8 inches tall and wearing a light-colored shirt and a camouflage baseball cap.
She left the bank on foot and fled in an unknown direction with an undisclosed amount of money, police said.
Danbury detectives are continuing to investigate the robbery and are awaiting surveillance footage from the bank.
Click here to follow Daily Voice Danbury and receive free news updates.