Judge Hilary Squires’ found Schabir Shaik guilty of corruption and fraud for paying kickbacks and bribes to Mr Zuma – a decision that threatens to derail his political ambition.
The case has proved to be one of the most sensational graft cases in post-apartheid South Africa, and was carried live on nation-wide television.
Although Mr Zuma wasn’t on trial, the verdict could be devastating for his political ambitions, with opponents calling for him to be sacked.
Analysts claim Shaik’s conviction could dash Mr Zuma’s hopes of succeeding President Mbeki when his second and last term in office ends in 2009.
The court found Shaik’s various companies paid Mr Zuma 1.2 million rand (A$230,000) to help secure lucrative business contracts.
“All of [Shaik’s] companies had been used at one time or another to pay Jacob Zuma in contravention of, the Corruption Act,” Judge Squires said.
“The evidence clearly shows a readiness in both Shaik to turn to Zuma for help and Zuma’s readiness to give it.”
The businessman was also found guilty of brokering an annual 500,000 rand bribe for Mr Zuma to ensure a French arms company was shielded from investigation into a controversial arms deal.
Shaik was ordered to pay 100,000 rand in bail and to hand over his passport pending sentencing on Friday.
In South Africa, a conviction on corruption charges carries a mandatory 15-year jail sentence.
A spokeswoman for Mr Zuma, who was out of the country on a visit to Zambia, said in a statement he was studying the judgement while government spokesman Joel Netshitenzhe said “the deputy president was not on trial.”
Mr Zuma, a 63-year-old an ethnic Zulu, has been deputy president since 1999. He’s seen as the frontrunner for the top job and viewed as a way of appeasing the country’s Zulu majority.