Incident comes after couple splashed around in Bernini fountain in Piazza Navona.
A man dressed in swimming trunks climbed between the statues of Rome's Trevi Fountain and dived into the waters of the Baroque monument before being detained by police.
The bather, described by Italian media as a 40-year-old Roman, ignored orders to get out of the fountain, leading a police officer to wade in and drag him out by the arm.
The incident took place just before 23.00 on Saturday night in front of onlookers who filmed the illegal dip on their phones, with footage shared on the Welcome to favelas Instagram page.
The man was reportedly fined €200 and handed a temporary ban or 'daspo' from returning to the UNESCO World Heritage site, according to Rome newspaper Il Messaggero.
However two hours later the man returned to the fountain for a repeat performance, jumping in from the same spot and being dragged out of the water a second time, by a different police officer.
In April two Dutch tourists were fined €1,000 for swimming in the Trevi Fountain, while on Friday night a man and a woman were filmed splashing around in Bernini's Fountain of the Four Rivers in Piazza Navona.
On Saturday Italian news outlets reported that the exterior of the Pantheon had been vandalised with graffiti that read "Aliens Exist", written in English with blue spray paint.
The police investigation into the incident was hampered after it was discovered that the city's security cameras guarding the site had long ceased to be functional.
Rome's tourism councillor Alessandro Onorato told Leggo newspaper on Monday that "more severe penalties" were required for those who damage the city's historic heritage.
"We need tourists who respect our city, who know how to experience it with the respect and attention it deserves" - Onorato said - "For us the tourist is sacred, but we need quality tourism that respects the city to the full."
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Rome police pull swimmer out of Trevi Fountain, twice
Piazza di Trevi, 00187 Roma RM, Italy
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