Katsav faces at least four years in prison. His lawyers have said they will appeal.
The rape case stemmed from when Katsav, 65, was minister of tourism before he became president in 2000. He was also accused of sexual harassment from his time as president.
They are the most serious criminal charges ever brought against a high-ranking Israeli official. Although Katsav is a member of Benjamin Netanyahu's Likud party, Israel analysts do not expect any backlash against the prime minister.
Most of the current trial was held behind doors. The verdict was broadcast live on all three Israeli television channels and trumpeted in banner newspaper headlines. Several dozen women demonstrated outside the courtroom against Katsav, holding signs saying "we believe you," referring to Katsav's former employee who brought the rape charges.
The court's criticism of Katsav was harsh, saying his version of events was "riddled with lies." Katsav had no immediate response to the verdict, although his son told Israel Television his father was innocent.
The verdict was the main topic of conversation in Israel today.
"This is a sad day for the state of Israel but a joyous day for Israeli democracy," Yonatan Livny, a Jerusalem lawyer, said as he got off his motorcycle. "It shows how the law can reach any echelon of society."
The presidency in Israel is a symbolic office, with the president often called "the No. 1 Citizen in Israel." Livny suggested that the president should undergo a confirmation hearing similar to officials appointed in the U.S.
Katsav, who was born in Iran and immigrated to Israel as a child, said he was being unfairly judged because he is Sephardic, a Jew from an Eastern country. The elite of Israel have traditionally been Ashkenazi Jews from Eastern Europe.
Katsav accused journalists of a "witch hunt" against him, but some Israelis felt the press was not vigilant enough.
"Before Katsav was elected, there were all kinds of rumors that he was a 'lady's man,'" Livny said. "The press knew these rumors and yet they were all afraid to pursue them and see if there was any truth to them."
But others blamed the media for judging Katsav before the facts were in.
Others said the verdict is a victory for women's rights.
"From today, every woman -- whether she has a doctorate or a fourth-grade education -- will know that there are some things no one has the right to do to her against her will," Neri Livneh wrote in the Haaretz newspaper. "No one can harass her any more without being called to account. Even someone who was president when the first complaint against him was filed was not able to evade investigation and trial, despite enormous efforts."
Katsav was forced to resign in 2007 just two weeks before his term was due to end. He originally accepted a plea bargain that would have him admit to lesser charges but would not entail a jail sentence. But he changed his mind and insisted he wanted a trial to prove his innocence.