The 38-year-old identified only as Roberta stunned lookers-on by stripping out of her clothes and swimming several lengths of the fountain before stretching out on the marble plinth at the base of a statue of Neptune.
The fountain was the site where Federico Fellini shot the famous scene featuring Anita Ekberg in La Dolce Vita
'The water is everyone's. I was hot,' Roberta said after her bathe.
But police took a dim view of her actions and frantically tried to cover her up with their helmets and arrested her.
It is illegal to swim in Rome's fountains and those who break the law could get a fine of £70.
But a spokesman for the police said she had been released with just a warning.
'She has been released but she has been warned she could face a charge of indecent exposure as this sort of behaviour is not allowed in the fountains of Rome or anywhere else in the city,' the spokesman said.
'She told us that she felt hot and that when she saw the water she lost control and dived in to cool off.
'Looking back now she is obviously a little embarrassed and wondering how to explain it to her boss when she goes to work.'
Una donna milanese, quarant'anni, si fa il bagno nuda a Fontana di Trevi
Thank heavens for the Italian newspapers. It would seem the only camera available was on a cellphone (IN ROME!!) - video here: