Investigators opened a criminal case after the man was pulled out of the Moika River, a waterway in the heart of the historic city, early Saturday morning (local time).
Professor Oleg Sokolov was arrested yesterday on suspicion of murder after he was dragged from a freezing river carrying two severed arms in a backpack.
Sokolov was awarded the Order of Legion d'Honneur, France's highest civilian decoration, in 2003 by France's then-President Jacques Chirac for his studies on Napoleon.
Sokolov's lawyer, Alexander Pochuyev, told Russian news agency Interfax that he had acted under "some sort of strong influence".
Oleg Sokolov, a prominent Russian academic specialising in Napoleonic history, confessed to murdering his young lover and former student Anastasia Yeshchenko and dismembering her body.
On Thursday, Sokolov allegedly killed his purported lover in an act of jealous rage, according to unconfirmed claims, cited by the media.
Reports say Sokolov liked dressing up as Napoleon and impersonating the former French emperor.
Vasily Kunin, who studied with the victim, blamed university management.
"There was a policy of hushing things up".
Media reports said that Sokolov also beat up and threatened to kill another woman in 2008 but was never charged.
"They did not pay attention to certain things", he told AFP.
On Twitter, screenwriter Andrew Ryvkin said Sokolov was one of his lecturers, describing the Saint Petersburg-based university as a place where "alcoholics" and "anti-Semites" felt at ease.
"As professor of modern history at the University of Saint Petersburg. awarded the French Légion d'Honneur, we would never have imagined that he could have committed this disgusting act".
Oleg Sokolov, 63, is one of the world's leading experts on Napoleon Bonaparte and was reportedly dressed up as the 19th-century French leader while trying to dispose of the evidence from his crime.
Issep was founded by Marion Maréchal, the niece of far-right National Rally party leader Marine Le Pen and a former lawmaker for the far-right National Front party.