Two female cashiers were terrorised with a black hand-held gun and ordered to fill a bag with cash, minutes before the bank was due to close at 4.30pm last Friday.
One of the women sounded the security alarm button next to her desk and, on hearing the alarms, the robber ran out and made his getaway.
Police said there may have been a vehicle waiting for him, although one wasn't seen at the time.
No shots were fired and no one was hurt thanks to the cashier's bravery, which was commended by an HSBC spokesman.
The spokesman added: "The staff were obviously very shaken and distressed by what happened but behaved admirably in the circumstances of this vicious incident."
Police are now appealing for witnesses who may have seen or heard anything at the time to come forward.
DC Richard Hayhoe of Crowthorne CID, said: "We are appealing to anyone who might have seen him — there is a bus shelter opposite the bank and people waiting there may have seem something. He would have been acting furtively and probably running.
"The cashier was very shocked and upset by what happened but no one was physically hurt."
The would-be thief was described as in his late 30s, between 5ft 8in and 5ft 10in.
He was wearing a blue woolly hat, green canvas jacket with brown lining, yellow gloves and faded blue jeans. He also wore glasses, which police say may have been a disguise.
Anyone with information can call the Crimedesk on (01753) 835555.