The corpse was found in the cellar by workmen surveying the site for a multimillion pound renovation, Le Monde reported.
The derelict mansion has stood empty for decades, but was bought by French businessman Jean-Bernard Lafonta at auction for €35.1m (£31.6m) – almost six times the reserve price – in January.
The hotel particulier or private mansion was once home to poet and playwright Francois Coppee.
It is just 700m away from the Hotel de Matignon, where the French PM Jean Castex lives, and backs on to the former residence of fashion designer Yves Saint Laurent.
Surveyors made the grim discovery on Rue Oudinot in the seventh arrondissement in February, but it has only just been made public, according to the French daily newspaper.
Police in Paris managed to identify the body as Jean-Pierre Renaud, a homeless alcoholic who died around 30 years ago.
His corpse was found with knife wounds and broken bones, prompting detectives to launch a murder investigation.
A police source told Le Monde: “Thirty years on, it appears to have been the result of a fight between homeless people.
“He had alcohol problems. The house seems to have been used as a squat when he died, but we can’t be sure he died here.”
The victim’s children have been informed, the source added.
Mr Lafonta did not comment and work is expected to start again on the mansion, which is protected by a strict preservation order, after the summer.
Source: Sky News