The accident happened on Friday afternoon on the R555 between Witbank and Ogies, spokesman Mmusi Moeti said.
Among those killed were four children. The driver of the truck was not injured.
Two people were taken to hospital in Witbank.
Moeti said human error was suspected to have caused the accident.
“MEC [Vusi] Shongwe has called for a high level probe into the accident and called for harsh punishment for those found to have caused the accident,” he said.
The taxi was travelling from Germiston in Gauteng to Ogies and the truck was travelling to Witbank.