Exactly seven years and one week since someone last pulled the stunt, a man poured dye into the Trevi basin in central Rome on Thursday afternoon.
He was stopped by a traffic warden and taken to a police station, Ansa news agency reported.
The previous incident, on October 19th 2007, was partly a protest against the cost of the Rome film festival, with the red waters symbolising the red carpet.
An Italian "vandalism artist", Graziano Cecchini, later claimed responsibility for the act, which he insisted used natural dye that wouldn't harm either the public or the 18th-century marble fountain.
In 2008, Cecchini struck another Roman landmark, rolling some 500,000 plastic balls down the Spanish Steps to symbolize each "lie told by a politician".
This year's edition of the film festival opened in Rome today, which suggests that the stunt is a re-run.
Looks like it is the work of 'neo-futurist artist' Graziano Cecchini. Police officer tells me at the Trevi: 'He's an idiot'. https://t.co/5TYXKHaJBx— Nick Squires (@NickSquires1) October 26, 2017