"If Zuma is elected, I don't want to be a member of the ANC anymore," he told Beeld.
Achmat, founder of the Treatment Action Campaign and currently also a champion for farmworkers' rights, said Zuma was robbing the country blind.
"The party needs a leader with integrity, someone above reproach, who doesn't hesitate to take action against corruption," he said.
Achmat said there was already court evidence linking Zuma to corruption.
"Schabir Shaik went to jail for his corrupt relationship with Jacob Zuma. Now there's the exposure of this whole Nkandla business … the president is indefensible, and the ANC is no longer the party I joined in 1990."
Achmat also criticised ANC secretary general Gwede Mantashe, saying that while ANC members were afraid of him, there was basically nobody outside the ANC who had any respect for him.
"The secretary general is supposed to be the person calling party members to order. He's supposed to take action against corrupt members, but he does it seldom, if ever." – Sapa