The storm that began late on Tuesday (local time) spared few areas, but the northwestern coastal region of Liguria and the central region of Tuscany were the hardest hit.
Six of the victims were in Borghetto Vara, a village in Liguria known for its grapes, wines and chestnuts. Roiling waters and mud tore through the village, 75km from Genoa and near the Cinque Terre, a popular vacation area. At least one building collapsed.
Another victim died while trying to clear gutters in the northwestern coastal town of Monterosso, built around a small natural gulf.
Two others were killed in Aulla in the nearby province of Massa Carrara, famed for its marble.
The head of La Spezia's provincial government, Marino Fiasella, told Sky TV24 that at least six other people were missing.
Flood waters swept away some roads and bridges, and several towns in Liguria remained cut off from the outside world. Authorities said major highways and the railway in Liguria would remain closed at least through Sunday.
Rome was under a flood alert but the storm caused little damage in the capital. A truck overturned in the storm, blocking the key Rome-Naples superhighway for three hours.
The Defence Minister said the military was being readied to help search for the missing and Italian news reports said up to 100 soldiers and two helicopters would take part.
