Nkosikho Mbele gave R100 ($6,80; £5,40) out of his own pocket to top up the tank of Monet van Deventer whom he feared would be left stranded on a notorious stretch of highway if she ran out of petrol.
On Friday, his story was a hit with South Africans who took to Twitter to sing his praises.
He asked her to return the money when she was in the area and did not ask for her contact details.
"I didn't even think about it... I did what was right," he told local radio station 702.
Ms Deventer was moved by Mr Mbele's kindness and started a crowdfunding campaign for Mr Mbele. The campaign has raised over K20 million (R400,000, $27,500, £22,000) in just two days.
Sowetan Live reports that the money will not be transferred to Mr Mbele's personal bank account, for fear of crime, but rather will be administered by BackaBuddy, a not-for-profit company, at his request.
The money will then be used to help pay for the education of Mr Mbele's children.