Barclays Bank in Wimborne Road, was raided at 10:30 BST on Tuesday.
The robber, who is thought to be in his early 20s, escaped on foot with an undisclosed amount of cash after threatening staff, Dorset Police said.
He is pictured wearing a motorcycle crash helmet with a red, white and black pattern on it.
The suspect is white, about 5ft 8in (1.7m), and was also wearing a black jumper, blue jeans, gloves, black trainers, police said.
Det Sgt Karen Penn, of Bournemouth CID, said: "This was a frightening ordeal for members of the public and staff who were in the branch at the time."
Police have appealed for any information about a P-reg car, possibly a silver, white or grey Renault Megane or Citroen Picasso which they believe the man got into after the raid.
CCTV images of a man who stole money from a bank in Bournemouth while wearing a "distinctive" crash helmet have been released.