The offenders struck as a security guard re-stocked a cash machine at a petrol station, raising a pistol to the man's head before taking boxes of cash.
They struck at what appears to have been a vulnerable moment in the operation because there is no indication that any security device, such as a dye spray, was activated by the crime.
Many cash delivery containers contain dye which is sprayed to mark the money inside if it is taken by criminals.
The gunman was joined by a second robber who helped carry the stolen money to a waiting Skoda Octavia car, driven by a third gang member, during the raid outside the Jet filling station in Doncaster Road, Mexborough.
They went to the carpark of the nearby Ferryboat pub and switched to a BMW X5 vehicle which was later abandoned at Hallgate, Mexborough.
Investigators believe a third vehicle may have been used but have so far been unable to trace witnesses from that point.
It was the second similar armed robbery in South Yorkshire this year, with another security van targeted outside a supermarket in Armthorpe, Doncaster, in March.
Both vehicles are believed to have been stolen in the North West and are currently being subjected to a highly detailed forensic examination.
If the offenders have left traces behind which can be identified as their DNA and they already appear on the national database, it would create an instant "hit", linking them to the crime scene.
The incident is being investigated by South Yorkshire Police's robbery squad and Det Sgt John McCurrie said: "We are still conducting house-to-house inquiries in the area where the second vehicle was abandoned and we would be very interested if anyone knows what happened to the offenders after that.
"The offenders have certainly used two vehicles and I suspect a third was involved," he said.
The 27-year-old guard was not physically hurt. However, he was badly shaken by the experience and has already been seen by specialist police officers who will make further checks on his welfare.
One of the robbers was wearing a Post Office-style uniform, which police believe may have been worn as an "excuse" to spend time loitering in the area immediately before the crime was committed.
Investigators are keen to trace anyone who remembers seeing a man wearing similar clothes in the area and have also appealed to businesses with private CCTV systems in the area to allow police to examine their footage.
Anyone with information should contact officers on 0114 2963308.