According to Timatic (the source used by most airlines), YES, a US citizen visiting South Africa DOES require an onward ticket, although there is an alternative.
This is the relevant section :
Immigration authorities may request visitors and transit passengers to
prove that they will depart from the country within the prescribed
period, by showing a return or onward ticket to their next
Unless stated otherwise, return/onward ticket is defined as:
a. International airline ticket (i.e. any types of airline tickets,
reservation confirmation, booking code etc.); or
b. Evidence of departing from the country by other means of
transportation (e.g. confirmation of joining a cruise, train, bus or
ferry tickets, proof of departing by private boat or plane, etc).
Technically there is a provision for visitors arriving without a return ticket, which is :
- Visitors arriving without a return or onward ticket will be required to pay a refundable deposit in lieu of ticket. Failure or
inability to pay this deposit will result in refusal of entry.
There is no mention of how much the refundable deposit is, or how it must be paid.
In general, your airline will confirm that you have an onward ticket before you board your flight, and it is possible that they will not consider the deposit clause when refusing you access, so it may not be a good idea to rely on that if possible.
You may be able to work around this by purchasing a fully refundable ticket for the return, and then cashing it in once you have arrived in the country. If you are caught doing this you can expect to be immediatley deported - but realistically the odds of that happening are small. If you have frequent flyer miles, it's worth checking if you can use them for such a flight - even if there is a fee for canceling, it's frequently less than for a normal ticket, and you won't have to tie up your money in order to buy a full fare/refundable ticket.
South Africa also requires that :
- Visitors are required to hold proof of sufficient funds to cover their stay and documents required for their next destination.
So if you book your onward ticket to a country other than the US, make sure that you have the required documents for that country as well!