The man, who appeared to be about 40 years of age, lunged at the painting with a screwdriver while shouting "s***head," according to the Luxembourger Wart.
A museum guard managed to stop the attack, but the vandalist successfully fled the museum.
Before starting his political career, Hitler aspired to be an artist, but was twice rejected by the Vienna Academy of Fine Arts, due to his lack of skill in drawing the human form. Before WWI, Hitler painted postcards and city landscapes to scrape together a meager living. He gave up painting after the war.
Museum curator Vittorio Sgarbi admitted to Italian news agency ANSA that the Hitler painting was "a piece of crap," but added that it shouldn't be censored.
"It's a painting by a hopeless man; it could have been done by Kafka, and says a lot about his psyche: here you do not see greatness, you see misery," he said. Sgarbi also denounced the attack, stressing that censoring even such "contemptuous" works would be akin to "reproducing the censorship and hatred expressed by the dictatorship."