Nkosazana Dlamini-Zuma, the former chief of the African Union, is a leading candidate to replace Zuma as head of the ruling ANC party in December and then as national president within two years.
President Zuma is widely seen as favouring Dlamini-Zuma, with whom he has four children, ahead of Deputy President Cyril Ramaphosa in the race that could split the African National Congress (ANC).
Dlamini-Zuma told AFP that she found it "offensive" to be seen as her ex-husband's chosen candidate to protect him from prosecution when he leaves office.
"I will not be elected by President Jacob Zuma," she said after delivering a speech in Johannesburg.
"If I am elected, I will be elected by South Africans, and they will have reasons why they elect me -- and that is why I find it offensive."
Dlamini-Zuma, 68, a former South African health minister, has not publicly declared her candidacy in the ANC leadership battle.
But she has campaigned across the country in recent weeks, drumming up support ahead of the party conference in December where the new leader will be elected by delegates.
President Zuma, who completes his final term in power in 2019, faces multiple court cases including almost 800 corruption charges that may be reinstated over a multi-billion dollar arms deal in the 1990s.
Dlamini-Zuma was an anti-apartheid student activist who fled into exile in Britain to qualify as a doctor before meeting Jacob Zuma in Swaziland.
"She is seen as one of the main contenders to take over from her ex-husband," the Institute of Security Studies think-tank said when she left the AU.
"It is alleged that she will protect Zuma and family members from prosecution for corruption if she becomes president."
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