The elderly man "was charged at by the animal" which launched a frenzied attack on him on a farm in the town of Cestas, south of Bordeaux, police said.
The man was found dead, with a swollen and badly bruised face.
Investigators believe the sheep knocked the man off balance, causing him to fall, and then trampled on him, inflicting more injuries.
Shortly afterwards, the sheep also charged at, and injured a local politician, who rushed to the scene after hearing the news of the attack.
The female councillor suffered a fracture and heavy bruising.
The police said it could see no other reason for the attack than "the animal's aggression".
The Sud Ouest newspaper reported that the sheep will be put down.