Over two million residents of the Veneto region took part in the week-long survey, with 89 percent voting in favour of independence from Italy.
The online vote, organised by local independence parties, is not legally binding but aims to galvanise support for a bill calling for a referendum.
Twitter lit up with excited separatists sparring with disparaging Italians from other regions who described the poll as "total madness".
Supporters say the new Republic of Veneto would be inspired by the ancient Venetian republic — a rich economic, cultural and trading power which existed from the seventh century until its fall to Napoleon in 1797.
The result was announced in Padua to a couple of hundred pro-independence campaigners who cheered and waved Venetian republic flags.
The Indipendenza Veneta party behind the bill says the separatist movement is fuelled by the government's apparent inability to stamp out corruption, protect its citizens from a damaging recession and plug waste in the poorer south.
The poll on plebiscito.eu asked inhabitants of historic cities such as Treviso, Vicenza and Verona whether — if the new republic was created — they would want to keep the euro and belong to the European Union and NATO.
But the region's president Luca Zaia told foreign journalists this week that the referendum bill — which must be approved by the regional council before it goes before the parliament in Rome — "still has some way to go".
Critics protest that an attempt to split from Italy could be unconstitutional.