Twenty-two people at the privately operated Cheneraie residence in Lherm, a town south of Toulouse, began showing symptoms including vomiting after dinner on Sunday, police said in a statement.
Four deaths were initially announced by officials, and a source close to the inquiry later told AFP that a fifth person had died.
About a dozen of the 82 residents at the site remain in serious condition.
The meals involved have been kept for analysis, the regional health agency said, and residents were being questioned about what they ate.
“We suspect food poisoning because these events occurred after the meal,” deputy prosecutor Marie-Paule Demiguel told BFM television, adding that the kitchens at the residence would also be investigated.
The grand-daughter of one of the woman who died, aged 95, said she had been served a Perigord salad, a regional speciality that includes duck, ham and foie gras, a duck liver pate.
“I still have the menu in my handbag and I know they had Perigord salad last night. What could that mean? Could it be the foie gras? They're going to do an autopsy in any case,” the woman told the Depeche du Midi newspaper.
The daughter of two other residents, who gave her name as Chantal, told journalists outside the home that “apparently the problem was with meals prepared specially for palliative care patients.”
The retirement home, open since 2006, operates under licence from the French government but is run by the private firm Korian, which claims to be the largest network of retirement homes in Europe, with over 800 sites in five countries, representing 78,000 beds.
“We learned yesterday of these four deaths and illnesses, and that there is suspected food poisoning, with an investigation and analyses underway, and we won't have any comment while awaiting the results,” Korian said in a statement.
“If food poisoning is determined, this situation nonetheless remains quite rare in a sector that is subject to strict oversight in terms of food security,” the AD-PA association of retirement home directors said.