The number of fatalities nearly tripled, rising from 133 to 366 Sunday, according to the civil protection agency, with most deaths occurring in the hard-hit Lombardy region in Italy's wealthy north.
The country now has the most deaths of any country outside China, and the second-most COVID-19 infections in the world, after the number of cases rose by a single-day record of 1,492 to 7,375.
South Korea currently has some 7,313 cases, and Sunday said its rate of infection was slowing.
Civil protection agency chief Angelo Borrelli said Italy was ordering 22 million surgical masks to help stop the spread.
Over 15 million Italians were adjusting to life in lockdown after the government issued a decree shutting off whole swathes of the north, including the city of Venice and the financial capital Milan.
It was not clear, however, how strictly the order would be enforced, or how authorities could prevent people from leaving.
Preventative measures were also imposed on the rest of the country, including the closure of all museums and monuments.