Cyril Ramaphosa is due to become South Africa’s president after embattled leader Jacob Zuma resigned.
Mr Zuma was under intense pressure from his own ANC party, which told him to step down or face a vote of no-confidence in parliament.
In a televised statement he said he was quitting with immediate effect but said he disagreed with the party’s decision.
Mr Zuma faces numerous corruption allegations but denies any wrongdoing.
As deputy president, Mr Ramaphosa automatically became acting president when Mr Zuma stood down.
A government statement says the National Assembly will elect a new president of South Africa on Thursday afternoon.
There is a renewed sense of hope as Mr Ramaphosa is taking over the reins of Africa’s most industrialised economy. Some will miss him though, pointing to achievements like announcing the abolition of fees for higher education, says the BBC’s Milton Nkosi.
Mr Zuma, a former member of the ANC’s military wing in the days of apartheid, rose through the ranks of the party to become president. He led the country for more than a third of its time after apartheid.
But he leaves office with several scandals hanging over him, and with South Africa’s economy in dire straits.
On Wednesday, police swooped on the Johannesburg home of the powerful and wealthy Gupta family.
The Guptas have been accused of using their close friendship with the president to wield enormous political influence. They deny all allegations of wrongdoing.