Phuket Police Station Deputy Phuket Police Commander Colonel Pirayut Karajadi said a man wearing a motorbike helmet and black clothing entered the bank at around 3pm today.
A video released shows him pointing a gun at terrified staff. One staff member hides under the counter while the other is held at gunpoint, and gets money to give to the robber.
Staff were unhurt during the incident.
Police say the man acted alone and he escaped on a motorcycle that had no registration.
Police said the man was believed to have driven through Sapam, and have cordoned off the area.