Thirty-one journalists were killed in the Philippines' Maguindanao massacre
Seventy journalists were killed in 2009, making it the worst year since records began 30 years ago, the Committee to Protect Journalists says.
A massacre of 31 journalists in the Philippines and other deaths took the total past the 2007 record of 67.
Some 150 journalists are currently in jail, including 23 in Iran where the CJP says the authorities have in effect criminalised journalism.
The group said online journalists were particularly vulnerable to repression.
According to its report, Attacks on the Press 2009, online reporters made up more than half of the news workers in prison worldwide.
As in the previous 10 years, China remained the world's worst jailer of journalists - with 24 being held.
China was followed by Iran, Cuba, Eritrea and Burma.
Speaking at a news conference at the UN, CPJ officials said international pressure was still the most effective way to combat both government repression and impunity for non-state players who attacked journalists, the BBC's Barbara Plett reports from the United Nations headquarters in New York.
The growth of new media - such as blogs and social networking sites - had created new opportunities to fight repression and censorship, they said.
But CPJ officials warned that states like China and Tunisia can sabotage such technologies and turn them against journalists.
It noted in particular that Tehran was using online social networks to go after dissidents and reporters.
"Creating vibrant and secure global media requires new strategic thinking to bring killers to justice, to reduce the number of journalists in jail, and to support reporters working in exile or in repressive environments," CPJ executive director Joel Simon wrote in the report's introduction.