The 13-year-old boy was taken in the eastern province of Mpumalanga while he was playing with friends near his home and was driven away by captors in a car.
"We are investigating a case of kidnapping that happened on Sunday in Witbank (town)," police spokesman Leonard Hlathi told AFP.
"There was a demand that was made that the parents should deposit cash in bitcoins," he said, declining to give further details.
Local media said the ransom note was left at the scene.
"We demand ransom of 15 bitcoins to be paid into the below bitcoin wallet address to secure your child's safe release - non-negotiable," read the reported note.
In March, US hackers demanding a ransom payable in bitcoin attacked computers of the Atlanta city government in the southern state of Georgia.
Police in South Africa, where violent crime is common, have reported a recent rise in kidnappings, although it is often wealthy business people who are targeted.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
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