It happened around 11:20 a.m. at the Wilmington Trust branch at 10 Fox Hunt Drive.
Police say the man approached a teller with a threatening note, demanding money.
The teller turned over an undisclosed amount of cash and the man fled the bank in an unknown direction.
No weapon was displayed and no one was hurt.
The suspect is described as a white man, about 40 to 50 years old, 5'6" to 5'8" tall and weighing 250 to 300 pounds.
Anyone who may recognize the suspect is asked to contact police at 302-834-2630 ext. 218. You can also contact Delaware Crime Stoppers at 1-800-TIP-3333.