The four men, who were kidnapped by al Qaida in the Islamic Maghreb (AQIM) in northern Niger in 2010, took off around 0600 GMT in a white government plane bound for Paris where they were expected to be met by President Francois Hollande.
Hollande was to greet the quartet on their arrival at an airport outside Paris after what he called "three years of suffering for the families who lived through a nightmare".
"I want to express my gratitude to Niger's president, who was able to obtain the release of our countrymen," Hollande added.
The exact circumstances of their release were not immediately clear.
But the French government said there had been no military assault to free them and that no ransom had been paid, while Hollande credited decisive help from his Niger counterpart.
Foreign minister Laurent Fabius told AFP the hostages had been freed in neighbouring Mali, and were in "very good shape".
"They have been hostages for three years and the nightmare is finally over," he said.
The four men appeared thin but otherwise in good health, an AFP reporter at the scene said.
In a brief remark to AFP at the airport, one of them, Thierry Dol, 32, said: "It was very difficult but it was the test of a lifetime."
Francoise Larribe, who was kidnapped along with her husband Daniel before she was freed in February 2011, said on learning of her husband's release: "It's an emotional wave, a tsunami."
"I have never lost hope, even though there were moments of dejection, fear and anguish," she said.
Dol, Larribe, Pierre Legrand and Marc Feret were kidnapped on September 16, 2010, from a uranium mining compound in Arlit, north-central Niger. They work for French nuclear giant Areva.
Speaking as he met the ex-hostages, Niger President Mahamadou Issoufou said his country had worked for their release, but provided no details.
"Since the kidnapping of the hostages three years ago, Niger has worked on obtaining their release. Now it's done," he said, congratulating the hostages for "regaining their freedom after months of difficult trials".
Legrand's mother, Pascale Robert, told BFMTV: "Now we're waiting for them to physically return, to see them, to touch them."
Negotiations in Mali desert
The news of their release came days after regional security sources in the town of Gao in Mali reported the presence of envoys in the Sahel "to speed up negotiations towards freeing the French hostages".
France had officially denied sending envoys.
According to a high-ranking Nigerien source, the four were taken to Niamey by a French plane from Anefis, in northeastern Mali near the Algerian border.
That was the site of final negotiations which included Mohamed Akotey, a former Tuareg rebel who is a senior executive with an Areva subsidiary in Niger.
The hostages were apparently held in different locations to prevent them being freed in any French assault, and were brought together just days before the release.
A Malian security source also said that "the final negotiations took place in the Malian desert", adding that "eminent Malians in the north provided timely assistance".
Mali's government welcomed the release but made no mention of any negotiations or whether it took part.
Three other people who were kidnapped at the time -- Francoise Larribe, a Togolese and a Madagascan -- were freed in February 2011.
AQIM had demanded at least 90 million euros ($124 million) for the release of the remaining hostages.
At least seven French hostages remain in captivity around the world, including two snatched in Mali, one in Nigeria and four in Syria.
AQIM grew out of a movement launched in the late 1990s by radical Algerian Islamists who sought the overthrow of the Algiers government.
The organisation allied with al Qaida in 2006 and has spun a network across tribal, clan, family and business lines that stretches across the vast Sahel region abutting the southern Sahara desert.