S&P lowered the country's local-currency rating one step to BB+, one level below investment grade, and placed it on a stable outlook.
S&P Global and Moody's were due to give their decisions on South Africa's investment status on Friday.
Standard & Poor's and Moody's are set to make their announcements on Friday.
A selloff of rand bonds - which comprise about 90 per cent of South Africa's outstanding liabilities - would raise borrowing costs for the nation as it sells more debt to plug a widening budget gap.
"By not downgrading the country further, Fitch is providing SA with an opportunity to address issues that can lead to an upward revision to the ratings".
"We think the government will attempt to introduce offsetting measures in an effort to improve budgetary outcomes, but these may not be strong enough to stabilise public finances, and may weaken economic growth further in the near term".
On November 23, Fitch Ratings had kept the sovereign ratings of South Africa at "BB+' with a 'stable" outlook.
"The review period may not conclude until the size and the composition of the 2018 budget is known next February. But, given insatiable demand for emerging market debt, there could be a natural underpin for even South African debt", said Lesiba Mothata, chief economist at Alexander Forbes.
South Africa's Reserve Bank will announce the decision on its main lending rate after 1300 GMT, and 26 out of 28 analysts polled by Reuters this month expected the bank to keep the rate on hold at 6.75 percent.
S&P decision will see South Africa excluded from the Barclays Global Aggregate index, whose inclusion criteria requires investment grade rating on its local currency debt from any two ratings agencies.
Jabu Mabuza, convenor of the CEO Initiative, said that all the negative ratings actions that the country has suffered "could and should have been avoided", had the required structural reforms needed to sustain inclusive economic growth been implemented in the interests of all South Africans.
Policy uncertainty, such as the introduction of the controversial new Mining Charter; the deterioration at major state owned entities; the fiscal slippage revealed in the October adjustments budget, and ongoing allegations of corruption all contributed to this environment he said.