The suicides came just four days apart. The second couple, aged 84 and 81, were found dead on Monday at their apartment in a building in the fashionable seventh district, police said.
"They committed suicide by swallowing medicines," the police source said. "They left a letter explaining their action."
The caretaker of the building told AFP their bodies were discovered by the maid. A neighbour described them as a "nice elderly couple".
Another neighbour said the woman suffered from cancer and walked with crutches. "But they went to the theatre and liked to go out," she told AFP.
On Friday, another couple -- both aged 86 -- took their lives in the luxury Le Lutetia hotel. Their bodies were discovered by room service taking up their breakfast.
They asphyxiated after putting plastic bags on their heads and were found lying hand in hand.
The couple, who met as students after World War II and had been together since, left a typewritten note claiming "the right to die with dignity."
Bernard and Georgette Cazes also asked their sole surviving son to campaign for the right to euthanasia after their death.
Socialist President Francois Hollande promised during his 2012 campaign to look into legalising euthanasia.
Assisted deaths are either legal or have been decriminalised in Switzerland, Luxembourg, Belgium and The Netherlands.
A six-month study on euthanasia last year said that active euthanasia -- an act by a third person intended to cause death -- should remain banned.
But it said the government should take into account public opinion on the subject. A poll by IFOP early this month showed that 92 percent of French people favoured euthanasia for patients suffering from terminal illnesses or facing great suffering.
Jean-Luc Romero, the head of the Association for the Right to Die with Dignity, told Le Parisien daily on Monday that only 10 percent of successful suicides in France involved the use of medicines, with the rest forced to take violent action to kill themselves.
"With assisted suicides one avoids the tragedies, and they can lead to a departure in a few minutes, calmly, without suffering and in the presence of loved ones."