The robbery happened at 2 p.m. in Northwest Fresno at the Westamerica branch on Shaw near Forkner.
The suspect told a teller, he had a weapon.
"(He) entered the bank with a note, demanded money from one of the tellers," said Lt. Mark Hudson. "At that point, he told them he had a gun. They gave him an undisclosed amount of cash."
The robber left on foot. He's described as 20-to-30 years old and wore a green long-sleeve shirt and a black beanie cap.
Police say he left behind a white dust mask.
Investigators are using the mask and surveillance video to track down the suspect.