Volpato's contract extension to 2031 was announced on Wednesday morning (AEDT), as a police matter over a positive drug test continues to cloud the 22-year-old's national team prospects.
The winger returned a positive roadside reading for cocaine after he was pulled over by police on the Anzac Bridge in Sydney on July 24.
He had been clocked driving at 109 kilometres per hour in a 60km/h zone.
An independent test at a private clinic returned a negative reading, before Volpato was sent for a secondary oral fluid test by police.
That procedure confirmed a positive result for cocaine, which resulted in a $722 fine.
Volpato has maintained his innocence and the result of the second test will be taken to court, as confirmed to AAP by his lawyer Paul McGirr.
A court date has yet to be decided, with the positive reading on Volpato's secondary test threatening to have major ramifications for his Asian Cup availability in January.
Football Australia (FA) has refused to comment on Volpato's test result and fine, noting "the matter is ongoing".
Sources have previously told AAP that Volpato could be hauled before FA's disciplinary and ethics committee, where the sport's governing body has the power to suspend him from national team action.
The Socceroos are next in action on November 14 (AEDT), when they tackle Qatar in Doha.
Volpato said he never knowingly ingested the substance.
The Sydney-born player was tested when he returned to Italy, with Sassuolo reporting a negative reading.
Volpato made his return from a hamstring injury in a 4-2 defeat to Frosinone last week, before Sassuolo released a brief two-sentence statement announcing the new contract.
"U.S. Sassuolo Calcio is pleased to announce the extension of Cristian Volpato's contract until June 30, 2031," the statement read.
"To Cristian, a huge good luck for his career with our colours."
Volpato was a late inclusion for the Socceroos' World Cup campaign earlier this year after switching his international allegiance from Italy on the eve of the tournament.
