Police were called to the Bristol Road South branch, in Northfield, at 11:45 GMT to reports a female member of staff was being held hostage in an interview room.
West Midlands Police said it is understood a firearm was produced, but no threats or demands were made.
They said the man, aged 60, surrendered to police at about 12:50 GMT.
Police said the suspect, who is believed to be from the local area and a customer at the branch, was taken to hospital for an assessment.
He is expected to be questioned later on suspicion of firearms offences.
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Officers, including armed police, contained the building and negotiators were sent to the scene as a cordon was put in place.
Samantha Lacey, 32, from Northfield, said she arrived at the bank to make a deposit but found the door was shut.
She said: "There were about six people outside. When I asked what happened they said a man walked in, pulled a gun out and told all customers to get out."
Ch Insp Shaun Phillips said: "Officers quickly surrounded the bank, set-up a street cordon and established phone contact with some staff members who were hiding in a back room.
"It's understood the man was with one female staff member, while other staff were lying low in another back room.
"No customers were caught up in the incident and no-one was hurt."
A man was arrested on suspicion of holding a staff member at gunpoint at a NatWest bank in Birmingham.