The committee released its annual Impunity Index, which calculated unsolved killings of journalists in the past 10 years as a percentage of the country's population.
Some countries, like Nepal and Russia, are showing improvement, while others are becoming more dangerous, the report said.
Nigeria was on the list for the first time this year, ranked 11th. To make the list, a country has to have had at least five unsolved killings.
"Investigations into these killings are usually carried out with sloppiness, and no real culprits are caught," said Ayode Longe, a senior officer with the Nigeria's Media Rights Agenda. "That has emboldened others to assault journalists, believing nothing would be done to them."
Other countries where violence directed against journalists appears to be on the upswing are Brazil, Pakistan and Somalia. Countries where conditions appear to be improving are Iraq, Sri Lanka, Mexico, Colombia and Afghanistan.