That sobering truth emerges in the updated "impunity index" produced by the New York-based press freedom watchdog, the Committee to Protect Journalists (CPJ).
The index, which calculates unsolved journalist murders as a percentage of each country's population, also found soaring impunity rates in Somalia, Pakistan and Brazil.
In those countries - plus Iraq, the Philippines, Sri Lanka, Mexico, Colombia, Afghanistan and Russia - journalists are murdered regularly and their killers go free.
"In countries where authorities fail to deliver justice in the killing of journalists, the result is more killings, while journalists try to survive by exercising self-censorship," said CPJ executive director Joel Simon.
"Nigeria's entry on the index suggests that violence is beginning to limit coverage of crucial issues, posing a grave threat to the country's democracy. The government must exert the necessary political will to solve these crimes."
Nigeria appears on the index — published to mark World Press Freedom Day tomorrow - for the first time. With five unsolved murders since 2009, Nigeria ranks 11th of the 12 worst countries for impunity.
It is one of two African countries on the list. The other is Somalia, where 12 journalists were murdered in 2012 despite relative calm in Mogadishu.
Deteriorating security, accompanied by a lack of justice, was also seen in Pakistan, where authorities have failed to prosecute a single suspect in the 23 journalist murders over the past decade.
In Brazil, where provincial reporters have proved especially vulnerable, the impunity rating has risen despite the government's stated commitment to justice.
Ten countries have appeared on the index every year since it was launched in 2008, highlighting the challenge of reversing entrenched impunity.
According to CPJ research, in Pakistan and the Philippines combined, at least nine witnesses and people connected to journalist murder investigations have been killed or died in suspicious circumstances in the past 10 years.
Simon said: "Governments that are committed to solving these cases must guarantee witness protection.
"A UN plan to combat deadly anti-press violence provides a pivotal opportunity for governments to take decisive steps to deliver justice."
A decade after the US-led invasion, Iraq remains the worst nation on CPJ's index, with more than 90 unresolved murders. CPJ found that journalist murders slowed there, as well as in Sri Lanka, Mexico and Afghanistan, but there are few successful prosecutions in any of these countries.
Colombia has had modest success in solving murders. Alarmingly, government and military officials are considered the leading suspects in 26% of murder cases on the index.
The index calculates unsolved journalist murders as a percentage of each country's population. It covers the period 1 January 2003 to 31 December 2012. Only nations with five or more unsolved cases are listed.
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