The limb - severed above the knee, apparently with an electric saw - was fished out of a tributary of the River Tiber, which flows through Rome, on Tuesday.
The leg had been in the water for less than two days, investigators believe.
Police believe the leg's owner was murdered and then chopped up. When, where and why the macabre deed took place still remain unknown.
The daily Il Messaggero identified the victim as Gabriele di Ponto, 36, who had a long list of convictions for drug-trafficking, armed robbery and violence.
He was also a member of far-right "ultra" supporters of the Rome football club Lazio, whose name was also tattooed on his leg.
He had recently married an Italian-Tunisian women, but her father told the newspaper she had left him after a month because he beat her.
"I have never come across anyone as nasty as him," her father told the daily.
Di Ponto carried out his first armed robbery at 18 and served several jail terms after that, even boasting of his time behind bars on his Facebook page.
"Hello to all prisoners," he wrote. "Better to be in a cell, silent, than to be without honour."
He had a limp after having survived being once sprayed with bullets, and this slight handicap allowed the police to identify him when he carried out robberies of pharmacies armed with a hatchet, reports said.
Di Ponto's relatives, worried that there had been no sign of him since late July, went to the morgue and identified his leg from five tattoos, press reports said.
DNA tests have been carried out in a bid to confirm the ID.
Rome prosecutors have opened a murder inquiry.