Roads and bridges and at least one house were washed away in mudslides and downpours, ANSA reported.
Bearing the brunt of the heavy rain was the country's northwestern region along the Liguria-Tuscany border, officials said. In La Spezia province, a house collapsed and killed three people.
"People are entering their homes through the balconies," Monterosso Mayor Angelo Betta said. "Everything is flooded. Monterosso is gone."
Mudslides blocked streets and collapsed bridges as water flooded rivers and streams throughout the area.
The military was placed on alert for rescue operations, ANSA said. Emergency personnel said they had difficulty reaching some of the flooded areas.
A highway in Liguria has been blocked in both directions since Tuesday, the news agency said.
"We spent the whole night in the bus," said soccer coach Ninni Corda, whose team couldn't reach a match because of the blocked road. "We witnessed scenes from the Apocalypse -- cars flipped upside down, one on top of the other."
Rail service in Liguria's coastal region was suspended as mud and debris covered parts of the tracks.
Heavy rainfall was expected further south this week as the storm front was forecast to move to Rome Wednesday.