Sokolov was detained on suspicion of murder while in hospital receiving treatment for hypothermia, but it was later announced that he had entered into a plea bargain agreement, according to his lawyer.
When the police rescued Sokolov from the river, they found severed arms in his backpack.
In 2003, then French President Jacques Chirac signed a decree awarding the historian the Order of Legion d'Honneur, France's highest decoration.
Russian Emergency rescuers and police investigators conducted searches on the Moika River, in Saint Petersburg, following the Sokolov's young lover and former student and dismembering her body in a grisly crime that sent shockwaves across Russia.
Oleg Sokolov, 63, was trying to dispose of the severed body parts of his girlfriend when he slipped and fell into the Moika.
Investigators did not name the man but police speaking to AFP identified him as Sokolov.
The historian has written several books on Napoleon and also acted as a historical consultant on several films.
After disposing of the corpse he reportedly planned to commit suicide at the Peter and Paul Fortress, one of the former imperial capital's most famous landmarks, dressed as Napoleon.
David's Evening Forecast - Blustery and cold conditions tonight
Wind gusts could reach 30 to 40 miles per hour on the Cape and Islands and 20 to 30 mph across the rest of the state. American GFS model projects light rain changing to light snow next Tuesday as Arctic air spills into the region.
Both he and Yeschenko enjoyed wearing period costumes, with Sokolov frequently dressing up as Napoleon.
Sokolov was a senior member of the Russian Military-Historical Society headed by Culture Minister Vladimir Medinsky.
Students at the university have since described Sokolov as a "freak", stating he would often do impression of Napoleon and would call his lover "Josephine".
Russian state news agencies reported that divers searching the Moika River as part of the investigation had discovered the remains of another person in a bag.
The arrested man is suspected of execute, the police said. It is unclear if the discovery is related to the death of Yeshchenko.
The BBC reported he was a member of France's Institute of Social Science, Economics and Politics, but they removed him from his position on the committee on Sunday.
"Sokolov acted under some sort of strong influence, for now we can only make assumptions", Pochuyev said.