The "shocking" incident happened at about 11.55 as the 57-year-old security guard approached the Tesco store in Annick Street in Shettleston.
The robber took the money box before making off in a small white car, later found burnt out nearby.
Police have appealed for anyone with information to contact them.
Officers said the white Hyundai i10 sped away from the scene of the robbery towards Springboig Road. It is believed there was a second man driving the vehicle, although police have yet to provide a description.
Det Insp Clark Hill said: "Due to the fact that it was believed that a firearm was present, our armed response units also attended but have since stood down.
"The suspect knew very well when the van would be there. It certainly wasn't done on the spur of the moment.
"With that in mind, it's possible that the car had been in the area either immediately prior to the robbery or indeed the past few days checking the area out."
No-one was seriously injured in the incident but a security guard, aged 37, from Tesco, who came to the victim's aid suffered a minor injury to his knee after being struck by the car, police said.
He was treated at the scene by paramedics and did not need to go to hospital.
Mr Hill added: "Although thankfully, no-one involved in this incident was seriously injured, this was a terrifying ordeal for the men involved. Also, for it to take place outside a busy supermarket in the middle of the afternoon is just shocking."
Officers have been at the scene carrying out inquiries and are checking CCTV both at the supermarket and in the street.
The suspect is described as being white 5ft 6in tall, of thin build and with a pale complexion. He was wearing a grey hooded top, a "hi-vis" yellow jacket and black jogging bottoms.
An armed robber dressed in a "hi-vis" jacket took at least Â£10,000 from a worker delivering cash to an ATM in the east end of Glasgow.