The robbery happened at the Wells Fargo Bank in the 2000 block of Concord Pike in the Fairfax community around 10:50 a.m.
Police say the man presented a note to the teller and apparently fled with an unknown amount of money.
The suspect is described as a tall black male, 40 to 50 years old with crooked teeth. He was wearing a tan hat and a blue jacket.
The suspect fled on foot.
Anyone with information is asked to contact the Robbery Unit at Troop 2 at 302-834-2630 Extension#6 or Delaware Crime Stoppers at 1-800-TIP-3333.