A first shipment of AstraZeneca/Oxford jabs produced in India is due to arrive on Monday, with injections expected to start two weeks later.
Health Minister Zweli Mkhize told The Sunday Times that 20 million additional vaccines had been ordered from US drugmaker Pfizer.
"These vaccines are secured and awaiting manufacturers to submit final agreements with details of delivery dates and exact amounts," said Mkhize.
The health ministry did not respond to several AFP requests for comment.
The new Pfizer order will complement 12 million vaccine doses from the WHO-backed Covax facility, nine million Johnson & Johnson shots and 1.5 million AstraZeneca/Oxford vaccines -- pushing the total supply secured so far to over 40 million.
South Africa aims to vaccinate 67% of the population, or 40 million people, by the end of the year.