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.

The meetings will take place in October at a mountain resort in North Korea.
The decision follows an agreement last month that de-escalated tensions sparked by a border mine explosion that injured two South Korean soldiers.
Thousands of Korean families have been separated with little or no contact since the war ended in 1953.
The highly emotional reunions of family member who have not seen one another in decades have been infrequent, and depend hugely on the state of relations on the peninsula.
The BBC's Stephen Evans in Seoul says the reunions, which started in 1988, used to be annual but have often been cancelled in recent years as relations frayed. The last round was held in February 2014.
About 66,000 South Koreans remain on the waiting list to see their relatives, many in their 80s and 90s, our correspondent adds.
The upcoming reunions, slated to be held at the Diamond Mountain resort in Mount Kumgang from 20 to 26 October, will see 100 people from each side selected.
Source: Korea Red Cross
The decision came after Red Cross officials from both countries held talks earlier this week at the border village of Panmunjom.
Communication between relatives across the border is highly restricted and almost non-existent. The two countries remain technically at war as the Korean War ended with an armistice.
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.
[NEXT_CONCEPT]
North and South Korea have agreed to hold rare reunions for families separated by the Korean War, according to Seoul.