At least 10 media workers have been killed over the past two years, while others have been abducted, tortured, or illegally detained, London-based Amnesty International said in a report.
Most of the victims are Tamil journalists working in the island's embattled north and east.
Sinhalese journalists working in the south face intimidation, particularly when reporting cases of graft, Amnesty said.
The rights group said those responsible are not punished by the government.
"Investigations have stalled and no one has been brought to justice for the deaths of media workers, including cases dating back to 1990," said Pia Oberoi, deputy director of Amnesty International's Asia-Pacific programme.
Tamil journalists working in conflict areas are under severe threat from security forces and armed groups who appear to be acting with the government's consent, the report said.
Amnesty also accused the Tamil Tigers of completely stifling dissent in areas under their control.
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