Matthew Todd Miller, 24, was arrested in April 2014, shortly after arriving as a tourist.
The US accuses North Korea of using Matthew Todd Miller and two other detained Americans as pawns in a diplomatic game.
The North Korean authorities have not specified the charges against Matthew Todd Miller, but they claim he tore up his visa and demanded asylum.
During the trial, prosecutors said Matthew Todd Miller admitted having a “wild ambition” to spend time in a North Korean prison so he could find out about the country’s human rights situation, the AP reports.
Notes produced in court also suggested he had become a fugitive because he was involved with WikiLeaks, the organization that has leaked US state secrets.
After a 90-minute trial, the sentence was handed down and Matthew Todd Miller was handcuffed and led from the room, AP reports.
The White House has described securing the release of Matthew Todd Miller and the two other American citizens detained in North Korea as a “top priority”.
In the past the US has been able to negotiate the release of American detainees.
Notably two journalists who were held whilst filming a documentary in North Korea were granted a “special pardon” after former President Bill Clinton travelled to the country.
The US has offered several times to send Robert King, its special envoy for North Korean human rights issues, to Pyongyang to discuss the detainees, but these visits have been cancelled by North Korea.
Matthew Todd Miller of Bakersfield, California, had been in custody since April 10.
Little information has been released about him, and the US State Department said this was partly because he had not signed a Privacy Act Waiver, which allows information about him to be released to the public.
According to KCNA, Matthew Todd Miller tore up his tourist visa on arrival in the country and shouted that “he came to the DPRK [North Korea] after choosing it as a shelter.”
In a brief interview with CNN earlier this month, attended by North Korean officials, Matthew Todd Miller said: “I will say that I prepared to violate the law of the DPRK before coming here.”
He also said he deliberately committed his “crime”, although he did not specify what he had done wrong.
In a recent interview with Associated Press, all three American detainees appealed to the US government to send a high-ranking representative to negotiate about their freedom.
State department official Daniel Russel told Reuters last week that the US found North Korean treatment of its citizens “objectionable and distressing”.
“This is the way that they play,” he said.
“They use human beings, and in this case Americans citizens, as pawns.”
Jeffrey Fowle came to North Korea as a tourist but was arrested in May for allegedly leaving a Bible in a public place. North Korea considers the distribution or spreading of Christian information as incendiary.
Korean-American missionary Kenneth Bae, who was arrested in November 2012, is serving 15 years in a labor camp after being convicted of trying to overthrow North Korea’s government.
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