Watson, a 74-year-old Canadian-American, was released on Tuesday after Denmark refused the Japanese extradition request over a 2010 clash with whalers. He was released after a high-profile campaign in his support.
French President Emmanuel Macron had pressed Danish authorities not to extradite the campaigner, who has applied for French nationality.
Watson landed at Paris-Charles de Gaulle Airport shortly before 3:00 pm (1400 GMT), an airport source said.
A smiling Watson was seen wheeling his luggage through the airport terminal as armed police looked on.
Supporters clapped and some shouted: "Bravo!"
One woman handed him a balloon. The activist left the airport without making a statement, only saying: "Thank you" in French and English.
"We didn't expect him to be released, especially before Christmas," said Gabriela Cabrera, a 49-year-old volunteer with Sea Shepherd, an organisation founded by Watson. She said she was delighted by what she described as "a miracle".
Another supporter, Laurence Huygues-Despointes, said she wanted to thank President Macron.
"We criticised him (Macron) a lot, but he vouched for him (Watson) and I hope he will obtain French nationality," said the 68-year-old.
Watson, who featured in the reality TV series "Whale Wars", founded the campaign group Sea Shepherd and the Captain Paul Watson Foundation (CPWF).
Adept at gaining publicity, he gained notoriety for "direct-action" tactics such as ramming vessels and using acoustic weapons, water cannon and stink bombs.
In the 2000s and 2010s, Sea Shepherd played a rough game of cat and mouse with Japanese ships as they sought to slaughter hundreds of whales every year for "scientific purposes".
Watson had been held in Greenland since July on a 2012 Japanese warrant, which accuses him of causing damage to a whaling ship and injuring a whaler.
- 'Feeling great' -
On his release on Tuesday, Watson had announced his intention to return to France, where his two young children attend school.
Watson's legal woes have attracted support from the public and activists, including prominent British conservationist Jane Goodall.
"It's been five months and this is the first day of freedom, so I'm feeling great," Watson told AFP in a video call shortly after his release from the Danish autonomous territory.
Tokyo accuses Watson of injuring a Japanese crew member with a stink bomb intended to disrupt the whalers' activities during a Sea Shepherd clash with the Shonan Maru 2 vessel in 2010.
Watson's lawyers have said they have video footage proving the crew member was not on deck when the stink bomb was thrown.
Supporters planned to stage a rally on Saturday afternoon to welcome Watson.
On Wednesday, Japan's government voiced dismay over Watson's release.
Supporters of whaling accuse their critics of double standards given the methods used to produce much of the millions of tons of meat from other animals consumed every year.
Paul Watson: eco-warrior on the high seas
Paris (AFP) Dec 17, 2024 - Veteran anti-whaling campaigner Paul Watson, released from detention in Greenland after Denmark refused a Japanese extradition request, has spent decades battling harpoonists and seal hunters in high seas confrontations.
For years a bete noire of Japan, one of the last three countries along with Iceland and Norway to practise commercial whale hunting, Watson was arrested on July 21 in Greenland, an autonomous Danish territory.
His first comment on being released was that his five-month detention had brought attention to "illegal" Japanese whaling.
Watson was arrested on a Japanese "red notice" international warrant when his ship was on its way to "intercept" a new Japanese whaling factory vessel in the North Pacific, according to the CPWF.
Japan accuses Watson of injuring a Japanese crew member with a stink bomb intended to disrupt the whalers' activities during a Sea Shepherd clash with the Shonan Maru 2 vessel in 2010.
Jean Tamalet, a lawyer for Watson, told AFP that "the fight is not over."
"We will now have to challenge the red notice and the Japanese arrest warrant, to ensure that Captain Paul Watson can once again travel the world in complete peace of mind, and never experience a similar episode again," Tamalet said.
The 74-year-old American-Canadian has received the support of Brigitte Bardot, the French screen legend turned animal rights activist, who accused the Japanese government of launching "a global manhunt" against Watson.
France's President Emmanuel Macron also pressed Danish authorities not to extradite the campaigner, who has applied for French nationality.
Watson devoted himself to saving marine life in 1977, forming what would become the Sea Shepherd Conservation Society. He was dismissed from the group in 2022 after infighting, which he said left a bitter taste. Some branches of the association, including in France, continue to support him.
Before then he had spent time with the Canadian Coast Guard and Norwegian and Swedish merchant marine ships.
- 'Pirate of compassion' -
Over the years he has become a media personality, appearing in the reality TV series "Whale Wars" and gaining notoriety for his direct-action tactics: chasing, harassing, scuttling and ramming illegal whaling and fishing vessels.
"We are pirates of compassion hunting down and destroying pirates of profit," Sea Shepherd's website quotes him as saying.
He uses acoustic weapons, water cannon and stink bombs against whalers.
Employing these methods, he has sunk more than a dozen boats and raided just as many.
As a campaigner, he has drawn on a degree in communications, galvanising support and funding from stars including longtime patron Bardot, Sean Penn, Pierce Brosnan and Pamela Anderson.
Born in Toronto in 1950, the eldest of seven children, Watson grew up in a fishing village in New Brunswick in eastern Canada.
His mother died when he was 13 and two years later he left home after falling out with his father.
His passion for whales was sparked in 1975, he says, when he was caught in a standoff with Soviet whalers and looked a dying whale in the eye.
"If we cannot save the whales, turtles, sharks, tuna, and complex marine biodiversity, the oceans will not survive," he said in one 2017 interview.
"And if the oceans die, humanity will die, for we cannot survive on this planet with a dead ocean."
- 'Eco-terrorist' -
Over 45 years, the intrepid Watson has carried out spectacular operations from Siberia to Iceland, Norway, the Faroe Islands and Japan.
With his crews he has saved thousands of whales and spotlighted the illegal activities of whalers.
In 2010 Sea Shepherd clashed with Japanese boats, leading to the sinking of the organisation's high-tech superboat Ady Gil in the remote Southern Ocean. He regularly says in interviews "we've never injured anybody".
At the time, Japanese ships hunted whales in the Antarctic and North Pacific for what it said were scientific purposes.
The white-bearded father of three claims in his biography to have co-founded Greenpeace in 1972 but said he parted ways with the group over arguments about protest tactics.
His ex-allies and the Japanese government label him an "eco-terrorist" because of his radical tactics.
He was detained for several months in the Netherlands in 1997 and lived in exile on the high seas from 2012 to 2014.