A police tactical response team traced the group to an address in Sebokeng, a township south of Johannesburg, as the suspects were on their way to attack a vehicle carrying money in the morning, police said.
"A shootout ensued," police spokeswoman Athlenda Mathe said. "A total of eight suspects were certified dead at the scene while the ninth suspect died in hospital".
Three others were wounded and had been taken for treatment at a nearby medical facility.
The group was believed to be behind a spate of cash heists on armoured security trucks in Johannesburg's Gauteng province and surrounding areas.
Six rifles, explosives "ready to be detonated" and four vehicles were seized at the scene, Mathe said.
Robbers often target vehicles transporting cash in crime-ridden South Africa.
Earlier this week, two bystanders were killed and one wounded after they were caught up in a robbery in Benoni, east of Johannesburg, and randomly shot at.
Nine Dead in South Africa Shootout
JOHANNESBURG - South African authorities said nine suspected robbers were killed in a shootout with police during a foiled cash heist on Thursday.