The island, famed for its Costa Smeralda beaches that draw royals, entrepreneurs and ordinary tourists alike during the dry, peak summer months, received more than 44 centimeters (17.3 inches) of rain in 24 hours Monday — half the amount it normally receives in a year — officials said.
Italy's civil protection chief, Franco Gabrielli, said the death toll may still rise as crews reach isolated areas in the countryside where some homes are submerged.
Transport was hampered by rivers of cocoa-colored mud gushing over roads, forcing the closure of several major thoroughfares, including a tunnel into the city of Olbia, according to Anas, which runs Italy's roads and highways.
Olbia Mayor Gianni Giovannelli said the city had been destroyed by the "apocalyptic" storm, with bridges felled and water levels reaching 3 meters (10 feet) in some places. He described the ferocity of the storm's rains as a "water bomb."
Sardinia Gov. Ugo Cappellacci said among the 17 dead was a family of four, reportedly of Brazilian origin, in Arzachena.
Local newspaper L'Unione Sarda said a policeman helping to escort an ambulance died when the car he was travelling in was submerged in the collapse of a bridge in Dorgali. In hard-hit Gallura, three people died after their car was swept away in the collapse of another bridge, it said.
Sardinia is the second-largest island in the Mediterranean and is one of Italy's autonomous regions. While it's known to tourists for its pristine beaches, the island's interior is known for its sheep and pastoral life. The island's people are famed for their exceptional longevity.