Now leader of the Five Star Movement (M5S) party, the former funnyman campaigned for the "no" vote which won Sunday's referendum.
Among other measures, voters were asked to approve an overhaul of the country's legislative process, namely the Senate -- one of Italy's two chambers of parliament -- which would see its membership cut from 315 to 100, reducing much of its power.
Outgoing Prime Minister Matteo Renzi lobbied hard for the changes, arguing they would benefit the country and help push his much-needed economic policies through Italy's bureaucratic red tape.
The resounding loss for Renzi now sets the stage for a general election -- and a possible win by Grillo's Five Star Movement (M5S) party.
So who is the leader of the anti-austerity M5S party and what does he stand for?
An anti-establishment entertainer?
Grillo, 68, founded the party in 2009 after making a name for himself on Italian TV in the 1970s and 1980s. He capitalized on his show-business bravado to promote an anti-establishment movement spanning both the left and right of politics.
"He's used to entertaining the public, so his speeches are also very entertaining," said Vincenzo Scarpetta,
senior policy analyst at think tank Open Europe.
"Even if you don't speak Italian, if you watch his body language you can see he's always very expressive and entertaining -- he moves a lot, running back and forth on stage."
Grillo has called for greater transparency in government, holding "V-Day" rallies -- short for an Italian expletive beginning with V -- which he says
also invoke the revolutionary spirit of "V for Vendetta."
"He's definitely a straight-talking guy," said Scarpetta.
Grillo's party's politics may be harder to pin down.
"It's difficult to place the movement on the political spectrum, precisely because they have refused so far to be categorized as either left or right wing," Scarpetta added.
What does Grillo stand for?
Ending the euro: The party's flagship policy centers around Italy abandoning the euro. "They have long promised to hold a referendum on Italy's membership of the single currency," said Scarpetta.
Opposing free trade: M5S also opposes major free-trade deals between the European Union and US (known as the Transatlantic Trade and Investment Partnership), and the EU and Canada (called the Comprehensive Economic and Trade Agreement).
"In that sense, they would be much more protectionist on trade policy than the current government," Scarpetta said.
Lower taxes for the poor: "The movement believes taxation for middle- and low-income people should go down, because these people have been stifled by successive governments raising tax," said Scarpetta.
Could he really be Italy's next prime minister?
Even if the M5S were to win a general election, Grillo -- who maintains a popular blog -- has long said he has no ambition to be prime minister.
A 1980 manslaughter conviction
from a car accident which killed three people has also thrown doubt over whether Grillo would even be eligible for the office.
So who from the party might be a candidate for the prime ministerial job?
"Luigi Di Maio is an obvious name," said Scarpetta, referring to the 30-year-old vice president of the Chamber of Deputies -- the other chamber of Italy's parliament.
"But remember this is also a movement that makes a big deal of transparency and accountability, selecting candidates by online surveys," he said. "I'm inclined to think they may do the same again."