Members of the protest party led by former comedian turned populist firebrand Beppe Grillo could propose their candidate for president through the party's website and a shortlist of the ten top names will be voted on next week.
The M5S as it is known by its Italian acronym said in a statement that the country's two main political forces — the centre-right and centre-left — wanted a president who would defend their interests and defend the status quo.
No other party is holding online voting.
"I believe the next president should not come from the political world or be someone who holds or has held a civil service job," said Grillo, who has proposed 87-year-old Nobel laureate playwright Dario Fo for the seven-year mandate.
He accused the parties of deciding on candidates in "secret rooms", saying his movement was having "a public and democratic consultation".
Among the possible names mentioned by M5S supporters on social media was Gino Strada, the outspoken head of the international medical charity Emergency, and Milena Gabanelli, an investigative journalist on public television.
Thursday's voting wraps up at 1900 GMT.
The shortlist will be voted online again on Tuesday and the winner will be the official presidential candidate for the movement.
A joint session of parliament is due to meet to begin voting for a new president next Thursday.