The vote's result suggests South Africa's years under a de facto one-party political system are nearing a close. The Democratic Alliance was once dismissed as a party for the white middle classes but appears to be doing a better job of winning a broader base of voters.
Analysts warn the result may further deter the ANC from pushing through economic reforms needed to boost South Africa's stagnating economy, ahead of a national election due in 2019. The World Bank sees South African gross domestic product growing by only 0.6 percent this year.
"Official election results show that support for the ruling ANC fell below 60 percent for the first time ever. Pushing painful structural reforms probably isn't the first thing that an unpopular government is going to do," BBH analysts led by Win Thin said in a research note.
"The knee-jerk reaction would more likely be to boost spending and handouts, which would pressure the country's ratings," they added.
BBH's base case is that South Africa will be downgraded to sub-investment — or "junk" — grade by one of the big-three credit ratings agencies by the end of the year.
At present, Fitch Ratings rates South Africa at BBB-, one notch above sub-investment grade.