Ethiopian wanted to baptise himself in "sacred waters" of Trevi Fountain.
Rome police detained a 38-year-old Ethiopian after he removed his clothes and jumped into the waters of the Trevi Fountain at around 10.00 on 23 January.
The man emerged from the freezing waters without resistance, in front of crowds of bemused tourists, and was promptly handed a €500 fine by police.
He justified his actions by saying he wanted to "baptise himself in the fountain's sacred waters", according to local media.
The man, resident in Mestre in northern Italy, was reportedly taken to hospital for a series of examinations.
This is the latest in a spate of incidents in recent years involving tourists misbehaving at the Baroque monument, from skinny-dipping to brawling over the perfect selfie.