St Peter's came sixth and Milan's Gothic cathedral followed in seventh place in the 2015 TripAdvisor Travelers' Choice Landmark Awards.
One reviewer wrote on June 4th that they were “not prepared for the beauty and size of the Duomo”.
“It was truly amazing and awe inspiring,” the reviewer said.
Reviewers were also full of praise for St Peter's Basilica.
“Utterly amazing. You don't need to be religious to be in awe of this beautiful place,” wrote one, while another said the Basilica was “the most important place to visit while in Rome”.
Photo: Jiuguang Wang
Cambodia's Angkor Wat temple topped the list, followed by the ancient Inca city of Machu Picchu in Peru, India's Taj Mahal, the Sheikh Zayed Grand Mosque in the United Arab Emirates and Antoni Gaudí's La Sagrada Família basilica in Barcelona, Spain.
The ranking is based on the millions of reviews posted by travellers on the TripAdvisor website over a 12-month period.