Prof Francesco Battaglia said that he had been threatened and his car vandalised after Mr Grillo criticised him for backing nuclear energy.
Mr Grillo, who is unlikely to go to jail, also faces damages of â‚¬50,000 (Â£36,000; $56,000).
He suggested people were afraid his party was getting close to government.
And he likened himself to two other historical figures who had spent time in jail, Italian anti-Fascist campaigner Sandro Pertini and South Africa's first black president, Nelson Mandela.
"If Pertini and Mandela ended up in prison, I can go there too for a cause I think is just and that has been supported by the overwhelming majority of Italians," he wrote on his blog.
Prof Battaglia said the maverick leader should carry out community service, just as ex-Prime Minister Silvio Berlusconi had done, and should take more care of what he said because of the large number of Italians who listened to him.
Mr Grillo publicly insulted the University of Modena and Reggio Emilia professor at a rally in 2011, after a TV appearance in which Prof Battaglia had supported nuclear energy.
"I'll kick your backside and throw you off TV," he said, urging his supporters to refuse to pay their TV licence fees.
Beppe Grillo, who was the driving force behind the Eurosceptic, anti-austerity protest party, is gradually handing over the reins of Five Star to a group of younger leaders.
The party is currently second in opinion polls behind Prime Minister Matteo Renzi's centre-left Democratic Party.
Beppe Grillo, who leads Italy's second biggest party, the Five-Star Movement, has been given a one-year jail term for slandering a science professor.