Christophe Deloire, head of Paris-based press freedom group Reporters Without Borders (RSF), died on Saturday aged 53, the organisation said.
Deloire died as a result of cancer, RSF said in a statement to AFP.
A former newspaper and television reporter, he had been at the helm of the watchdog group since 2012, transforming it "into a global champion for the defence of journalism," the statement said.
"Journalism was his life's struggle, which he waged with unshakeable conviction," said Reporters Without Borders, which is widely known by its French initials RSF.
In a tribute on X, formerly Twitter, French President Emmanuel Macron said Deloire "had journalism at his heart."
He said Deloire had fought "tirelessly" for freedom of information and democratic debate.
"His universal fight remains ours," he added.
The New York-based Committee to Protect Journalists said on X it was "deeply saddened" by Deloire's passing.
Phil Chetwynd, Global News Director of Agence France-Presse, said Deloire "was an indefatigable supporter of press freedom".
"He had immense energy and charisma, and was such a powerful advocate for our profession at a time when our work and our values are increasingly being called into question," said Chetwynd.
The activist group, launched in 1985 in the southern French town of Montpellier by four journalists, has become a thorn in the side of autocratic and despotic regimes around the world which would prefer to muzzle the media.
It has repeatedly condemned press freedom restrictions in Russia since Putin launched the war against Ukraine in 2022.
The group also launched a package of satellite news channels catering to Russia, much of it produced by exiled Russian journalists.
Emily Wilkins, president of the National Press Club in Washington, said: "Our hearts go out to Christophe's family at this time and to all in the RSF community and indeed to everyone around the world who has been touched by his compassion and inspired by his leadership."
© Agence France-Presse
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