Grillo's Five Star Movement, which campaigns on a variety of environmental and anti-corruption issues, is taking part in local elections in Sicily scheduled for October 28 to vote for a new regional governor and assembly.
The distance swum by the 64-year-old former comedian was three kilometres (1.9 miles) at one of the closest points between the mainland and the island.
He swam from the village of Cannitello in Calabria to Messina in Sicily under heavy rain and accompanied by a flotilla of fishing boats with media.
Grillo has been criticised for his populist rhetoric, including questioning whether Italy should be a member of the eurozone and calling for a trial against corrupt politicians similar to the one against Nazi war criminals.
Grillo's movement did well in local elections earlier this year and scored 15 percent in terms of popularity in a national poll published last month.
The last governor of Sicily, Raffaele Lombardo, was forced to resign in July because of a crisis in the public finances of the autonomous region which put it on the brink of bankruptcy and a scandal over massive waste of public funds.