Its annual World Press Freedom Index warned of the "highly toxic" media-bashing of Trump's election campaign and Britain's Brexit referendum.
The situation is at "a tipping point," it said bluntly.
Media freedom is being undermined by the rise in surveillance and of authoritarian strongmen across the globe, the watchdog said.
The US and Britain both slipped two places in the index to 43rd and 40th, according to the Paris-based monitoring group, known by its French initials RSF.
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"Media freedom has never been so threatened."
Liberty of the press is in peril or in a "very serious situation" in 72 countries including Russia, India and China, it found.
"Attacks on the media have become commonplace and strongmen are on the rise. We have reached the age of post- truth, propaganda, and suppression of freedoms -- especially in democracies," the report said.
"Donald Trump's rise to power... And the Brexit campaign were marked by high-profile media-bashing, a highly toxic anti-media discourse that drove the world into a new era of post-truth, disinformation and fake news," it added.
The nationalist government in Warsaw was accused of "turning public radio and TV stations into propaganda tools" and of trying to throttle independent newspapers.
"Media freedom has retreated wherever the authoritarian strongman model has triumphed," it warned.
Turkey "swung over into the authoritarian regime camp" after the failed coup against President Recep Tayyip Erdogan in July, it said, adding bitterly that it "now distinguishes itself as the world's biggest prison for media professionals."
Seven places ahead of Turkey, "Vladimir Putin's Russia remains firmly entrenched in the bottom fifth of the index," in 148th place, it added.
"Where will this downward spiral take us?" he asked.
The RSF index found that in the past year nearly two thirds of the countries had registered a deterioration in their situation, while the number of countries where the media freedom situation was "good" or "fairly good" fell by more than two per cent.