"Your president is corrupt. Your president has committed treason," Zuma told a news conference in Johannesburg -- his first since he ended his 15-month jail term earlier this month for contempt of court.
He was reacting to allegations earlier this year that Ramaphosa concealed a multi-million-dollar cash heist at his luxury farmhouse.
"No president should conduct private business while in office. It is inconsistent with the oath of office taken by (the) president," he said.
"Our country's problems are too big for a president who is busy hustling on the side."
South Africa's parliament this week opened a probe on whether the farm saga renders Ramaphosa impeachable or not. The police is also investigating the case, but Zuma said there was "silence of the many criminalities against the current president".
The case has piled unprecedented pressure on Ramaphosa, who came into office on the promise of busting graft.
The president has acknowledged a burglary but denies the accusations of kidnapping and bribery, saying he reported the break-in to the police.
He has also disputed the amount of money involved, and said the cash came from legitimate sales of game from his animal-breeding farm
The scandal erupted in June after ex-national spy boss Arthur Fraser filed a police complaint, alleging that robbers broke into Ramaphosa's Phala Phala farm in northeastern South Africa, where they found and stole $4 million in cash hidden in furniture.
Former president Thabo Mbeki also bemoaned the state of the ANC ahead of the December meeting.
"Our president is under a lot of pressure... around the matter of Phala Phala farm," he told a separate meeting in Johannesburg.
Mbeki went on to question the quality of leadership of the ANC, claiming some leaders occupying the top six jobs in the country are not qualified to do so.
Zuma last month he said he was ready to make a political comeback at the ANC party's internal conference.
On Saturday he said if the ANC asked him to run for one of the top party positions, "who am I to say no?"