The agency downgraded the country's sovereign debt rating from 'BBB-' to 'BB+' but kept the outlook stable.
It said the downgrade reflected the view that "recent political events, including a major cabinet reshuffle, will weaken standards of governance and public finances."
Already earlier this week, another rating agency, Standard & Poor's, also reduced South Africa's rating to sub- investment, as the country's currency, the rand, slid after President Jacob Zuma ousted his finance minister, Pravin Gordhan.