The porcelain urinal was slightly chipped in the hammering, which took place Wednesday during the final days of a Dada exhibition at the Pompidou Center.
The artist, Pierre Pinoncelli, 77, who urinated into the same urinal and also struck it with a hammer at a show in Nîmes, France, in 1993, has a long record of organizing bizarre "happenings."
Police officials said that he once again claimed that his action was also a work of art, a tribute to Duchamp and other Dada artists who had made their name by challenging the very definition of art.
"Fountain" itself was rejected for being neither original nor art when Duchamp offered it for the first exhibition of the Society of Independent Artists in New York in 1917. That version of the urinal, displayed upside-down and signed R. Mutt, was subsequently lost.
The Pompidou’s "Fountain" is one of eight signed replicas made by Duchamp in 1964.
The vandalism again raised questions of how valuable works of art can be protected in museums that have millions of visitors every year.
For example, the canvases of most paintings on display today are shielded by glass. At the Louvre, "Mona Lisa," which was stolen in 1911 and struck by a stone in 1956, is now in a sealed enclosure behind glass that is 38.5 millimeters thick, or 1.5 inches.
But Pinoncelli’s new attack also spotlighted a continuing debate around the question, "What is art?"