Hassan Ruvakuki was arrested in November, accused of involvement in a deadly gun attack by militants on the eastern town of Cankuzo that was launched from neighbouring Tanzania.
The prosecution alleged he was complicit in the attack because he had travelled to Tanzania earlier that month and interviewed the rebel group's proclaimed leader, a former Burundian police officer, Reuters reports.
"This is a shocking and shameful verdict. He did his job of investigation to inform Burundi's public on what was going on in the country," said lawyer Onesime Kabayabaya in reaction to the sentence handed down late on Wednesday.
He said Ruvakuki would appeal the conviction.
Thirteen alleged rebel fighters were also sentenced to life in prison by the Burundi court for their role in the attack.
Journalists in Burundi have been targeted in a wave of detentions in the past two years. Human Rights Watch said last month the government tried to restrict independent media and civil society efforts to denounce ongoing violence in Burundi.
Regional journalists' unions condemned the sentence handed down to Ruvakuki, who worked for Radio France International's Swahili-language service and local broadcaster Bonesha FM.
"The arrest and the whole trial were politically motivated and the verdict is a great affront to press freedom," Omar Osman, secretary general of the Eastern African Journalists Association, said in a statement late on Wednesday.
Alexandre Niyungeko, president of the Burundi Journalists' Union, said he was dismayed by the verdict against Ruvakuki.
Deadly clashes last year between security forces and former militia fighters rocked Burundi which had enjoyed relative peace since the Hutu FNL rebels laid down arms and joined the government in 2009 after two decades of civil war.
But violence intensified following elections in 2010 that were widely boycotted by the opposition.
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