Osizweni police station Inspector Sydney Mtshali said he broke down a padlocked door.
"I found the young boy tied with chains (on) his hands and legs, like a prisoner," he said.
"He was being kept at a place with chickens.
"The boy was scared, shaking, afraid and dirty. His condition was bad.
"From what the community members have explained (to me), this was not the first time the old lady had locked up the boy," he said.
A social worker had taken the boy to hospital.
The visibly traumatised boy said he had never been to school.
"I (long) to go there one day," he said.
"I have just seen some boys and girls of my age in their nice uniforms on their way to school.
"Gogo has always been promising to take me to school, but I don't (know) why she has chained me like this today. When she left in the morning, she promised me that she would buy me shoes."
When asked if his granny had chained him before, the six-year-old said: "Gogo beats me up with anything, even with a big stick or a pipe. But she has never chained me like this."
The 72-year-old woman is believed to have adopted a boy aged seven.
Mtshali said the six-year-old had been in an orphanage before he was taken in.
Approached for comment, the foster parent said: "Look, my boy, this child is very troublesome.
"I decided to lock him up because I did not want him to leave."