The pontiff insisted on leaving to Vatican to visit the store, despite the optician offering to come to him.
Francis spent around an hour choosing his new frames before praying with store workers.
He was joined by an assistant, a bodyguard and several police officers.
Tourist Daniel Soehe spotted the pope: "I told my father, 'Hey, that was better than going to St Peter's dome: Seeing the Pope in a shop trying on new glasses'," he told AP.
Pope Francis has always said he would like to be able to walk freely around the streets.
Before his election as pope he used public transport and walked most of the way to the Papal Conclave.