The man entered the bank at about 11:10 a.m. and presented a note stating he was robbing the bank and demanding cash, police said. No weapon was displayed.
He fled the bank on foot with an undisclosed amount of cash, and no vehicle was seen.
The man is described as a black male, age 30 to 40, 5 feet 10 inches to 6 feet tall, weighing 160 to 175 pounds, according to police.
He was wearing a light colored long-sleeve shirt, blue or gray pants and sunglasses and spoke with what police described as a Jamaican accent.
No one was injured.
Anyone with information is asked to contact the Pleasanton Police Department at 931-5100.