The government announced the rule on July 22. Some 50 million of Italy's 60 million residents had downloaded the certification by late July.
Vaccine certificates issued by the United States, Canada, Japan and Israel will be accepted for tourists arriving from those countries.
Along the sidewalk flanking Vatican City's walls, visitors to the Vatican Museums - one of the world's most popular attractions - got ready to show their cellphones with their QR certificates to staffers at the entrance. The line moved briskly.
Visitors from France found the new Italian system familiar. Their country has already introduced entrance requirements even tougher than Italy’s since they also apply to outdoor dining,
“It is good for everybody’s safety. It is positive for the economy, too,'' French tourist Alexine Prentignac said.
While many find it convenient to flash their Green Pass on a phone, paper certification is acceptable in Italy. The Vatican Museums website cautioned visitors to have an identity document handy so staff could “verify actual ownership” of the Green Pass. For anyone unwilling or unable to comply, the website offered instructions on how to request a ticket refund.
At Pompeii, one of Italy's most-visited tourist sites, officials teamed up with the city of Naples to offer coronavirus tests during the opening hours of the sprawling park featuring the ancient Roman city's ruins. For now, the tests are being given on an experimental basis, free-of-charge.
Business owners quickly wove the change into their customer routines. Fabrizio De Falco, the owner of a cafe near St. Peter’s Basilica with both an outdoor terrace and indoor tables, also regarded it a necessary adjustment during the pandemic.
“It is one more duty. It is an additional workload, but it is worth it if it will get us out from this situation,'' said De Falco.
The Green Pass rule applies to those 12 or older, since children under that age aren't eligible to receive a COVID-19 vaccine in Italy.
Much of leisure life in Italy during warm months is spent outdoors. Gyms, cinemas and theaters frequently close in August since staff and clients opt for vacations during the month. With air-conditioning still a relatively new phenomenon for many businesses, dining outdoors at trattorias and cafes is a summertime tradition in much of the nation.
Right-wing leader Matteo Salvini, whose League party is part of Italy's coalition government, had voiced worry that any Green Pass requirements for long-distance travel would discourage domestic and international tourism. The tourist industry accounts for 13% of Italy's GDP.
On Thursday evening, Premier Mario Draghi's Cabinet approved a new rule making such certification obligatory starting on Sept. 1 for those taking flights, high-speed or inter-regional trains or ships sailing between regions. An exception was made for ferries which frequently ply the narrow strait between Sicily and Calabria, a route taken by many commuters and students.
AP journalist Paolo Santalucia contributed from Vatican City.