Zuma won the race to lead the ruling African National Party in December, making him frontrunner to succeed Thabo Mbeki as the president of Africa's biggest economy.
However the left-leaning Zuma's trade union links are causing unease among investors and businesses who fear a Zuma-led government could reverse the liberal reforms of the past decade. Zuma is also awaiting trial for charges he denies relating to money-laundering, racketeering, fraud and corruption.
Fitch Africa analyst Veronica Kalema told a conference: "Political uncertainty has risen, because of the left-leaning members of the ANC and Zuma's corruption trial in August."
Any changes in rating would have to await the trial and more clarity on policy changes, she added.
South Africa is rated 'BBB+' by Fitch which placed it on positive outlook in July 2007.
Kalema said despite political uncertainty, South Africa is showing a steady improvement in macroeconomic fundamentals, with declining public debt, a build-up in reserves, modest external debt and firm economic growth.
Zuma has vowed to continue to centrist policies that have fuelled South Africa's economic boom.
Rival agencies Moody's and Standard & Poor's rate South Africa 'BBB+' and 'Baa1'.