Polls opened at 7:00 am (0500 GMT) and will close at 9:00 pm. There are 27 million registered voters.
High unemployment (32.9%), rampant crime, corruption scandals, regular power cuts, and water shortages are the major issues. The economy grew only 0.6% in 2023.
This is the first time since the end of apartheid in 1994 that the ANC could lose its outright majority, according to opinion polls.
The ANC is projected to win as little as 40% of the vote, down from 57% in 2019. If it wins fewer than 201 seats, a coalition will be necessary.
The Democratic Alliance (DA), led by John Steenhuisen, aims for clean governance, privatisation, and deregulation. Steenhuisen claims no party will secure an outright majority.
Cyril Ramaphosa, seeking a second term, highlighted progress in fighting corruption and improving electricity supply. He promised universal credit and health coverage.
Full results are expected by the weekend, with potential for significant political shifts.