Tashi Wangchuk was featured in a New York Times documentary that followed him on a trip to Beijing, where he attempted to get Chinese state media and courts to address what he describes as the diminishing use of the Tibetan language.
A court in Yushu Tibetan Autonomous Prefecture in the northwestern province of Qinghai sentenced him on Tuesday morning, according to his lawyer.
The court could not be reached for comment.
Tashi had been detained in his home town of Yushu without access to his family since January 2016, not long after the New York Times published its story and documentary video about his activism.
"Tashi plans to appeal. I believe he committed no crime and we do not accept the verdict," lawyer Liang Xiaojun told AFP.
Tashi had pleaded not guilty at his trial in January.
But nearly every case that goes to trial in China -- especially on sensitive state security issues -- ends with a guilty verdict.
Liang told AFP the short documentary was the main evidence used by the prosecution.
In the video, Tashi complained of a "systematic slaughter of our culture".
In the New York Times stories Tashi notably said he wants to use Chinese law to build his case and praised President Xi Jinping.
Beijing says it "peacefully liberated" Tibet in 1951 and insists it has brought development to a previously backward region.
But many Tibetans accuse it of exploiting the region's natural resources and encouraging an influx of the majority Han ethnic group which critics say is diluting the native culture and Buddhist faith.
China's constitution protects free speech but critics say in reality there is little room for any opinions that challenge government policies. Rights groups have accused Xi's government of an escalating crackdown on expression.
Amnesty International said the sentence was "a gross injustice".
"He is being cruelly punished for peacefully drawing attention to the systematic erosion of Tibetan culture. To brand peaceful activism for Tibetan language as 'inciting separatism' is beyond absurd," said Joshua Rosenzweig, its East Asia research director.
"The documentary underscores that Tashi Wangchuk was merely trying to express his opinions about education policy through entirely legitimate means," Rosenzweig said in a statement.
The video ends with Tashi discussing the many Tibetans who have self-immolated in protest at China's policies over the years, while adding what he would do if he is "locked up or they force me to say things against my will".
"I will choose suicide," he said.
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