A man entered the store on Balmoral Place between 07:30 and 07:45, threatened a 33-year-old member of staff with a weapon and attacked him.
The suspect fled with a four-figure sum of cash, cigarettes and the wallet and mobile of a 61-year-old customer who came into the shop and intervened.
Police described it as a "shocking" and "vicious" attack.
The suspect was described as white, in his early 20s, wearing black trousers, a black top, light pink trainers and a black scarf covering his face.
He was also carrying a black backpack which may have had pink markings on it.
Det Con Andrew Hope said: "This was a shocking attack. It was vicious and the member of staff sustained a serious head injury during the attack.
"He was taken by ambulance to the Borders General Hospital where he was treated and has since been released.
"The suspect had no thought for the staff member or the customer who thankfully was not injured in the incident.
"The Post Office is a busy place, as is Gala Park where the suspect made off towards, so I would appeal to anyone who saw the attack and who hasn't already spoken to police, or anyone who can assist officers with their inquiries to contact us."
A shop worker has been seriously injured during a raid on a post office in Galashiels in the Scottish Borders.