Three activists from the Italian anti-climate change organisation Last Generation (Ultima Generazione) poured a vegetable-based carbon liquid into the landmark 17th-century fountain, known to Romans as La Barcaccia, before being escorted away by police.
The fountain, in the shape of a boat, was designed by famed Italian sculptor Pietro Bernini.
Popular tradition has it he was inspired by the discovery in 1598 of a boat in the square after it was washed inland by a flooding Tiber river, Last Generation said.
Turning the water black “foreshadows the ‘end of the world’ scenario we are heading for, as we increasingly step on the accelerator: drought alternating with devastating floods, which will put an end to life on Earth, along with heat waves,” it said in a statement.
Last Generation began carrying out peaceful but disruptive protests in Italy last year ahead of the general election, urging politicians from all parties to make climate change their priority.
The protests in Italy are part of a series of actions across Europe to focus attention on climate change.
Activists have thrown soup, cake, mashed potatoes or washable paint at heritage and culture sites and artworks in museums.