According to local media, the tourists and locals gathered at the monument applauded his swim, many using their mobile phones to record the scene.
The police officers on duty however were less impressed. They covered the man in a towel before taking him to the police station.
There, the culprit reportedly tried to pass himself off as a foreigner before police established that he was a native Italian, according to local paper Roma Today.
He has been charged with resisting arrest and obstructing justice, in addition to the crime of trespassing in the 18th-century fountain. Checks are being carried out at the Trevi to see whether any damage was caused by the stunt.
The man is led away by police officers. Photo: Rome police
People - usually tourists - regularly trespass in the fountain, often in an attempt to recreate a scene from Federico Fellini's 1960 film 'La Dolce Vita' in which the late Swedish actress Anita Ekberg jumps into the Trevi.
In 2015, the authorities grew so tired of the illegal dips that they doubled fines for transgressors to €400.
But the increased penalty hasn't been enough to deter everyone.
Police presence has been stepped up at the landmark after its reopening last November, following a 2 million euro restoration. Fashion house Fendi funded the 17-month project, just one of a string of monuments to be spruced up thanks to money from private businesses.
The fountain, commissioned by Pope Clement XII in 1730, is the end point of one of the aqueducts that supplied ancient Rome with water and was last restored 23 years ago.
The Acqua Vergine runs for a total of 20 kilometres and ends up in the fountain, where tourists can drink it from a special tap tucked away at one side of the monument.
Legend has it the water source was discovered in 19BC by thirsty Roman soldiers guided to the site by a young virgin - hence the name, Virgin Waters.