A number of people are believed to be missing after rivers burst their banks, sweeping cars away and causing bridges to collapse.
The worst-hit area appears to be in and around the north-eastern city of Olbia.
Hundreds of people across the Mediterranean island were moved from their homes.
“We’re at maximum alert,” Giorgio Cicalo, an official from Sardinia’s civil protection authority, told Italy’s Rai TV.
“We haven’t seen a situation as extreme as this, perhaps for decades. Especially because it’s been across the whole island.”
Sardinian Governor Ugo Cappellacci told Italian TV that Cyclone Cleopatra had claimed the lives of at least 14 people.
A Brazilian family of four died in Arzachena, in the far north-east of the island, the Nuova Sardegna newspaper reported.
Three people died when a road bridge collapsed on to their car near Olbia, according to local media.
In a separate incident, a mother and her daughter were found dead in their car after it was swept away by floods.
Among the victims was a police officer who died after a bridge collapsed.
Olbia Mayor Gianni Giovanelli was quoted by Sky TG24 as saying that the city had been hit by an “apocalyptic”‘ storm.
Meanwhile Gov. Ugo Cappellacci told La Stampa newspaper that thousands of people had been affected.
“Unfortunately the situation is tragic,” he said.
Cyclones are extremely rare in the Mediterranean.
Some city residents used social media to offer shelter to those forced out of their homes.
The storms also caused extensive damage to farms on the island and disrupted a number of flights to and from mainland Italy.
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