A Bangkok military court convicted the 34-year-old of 10 counts of lese majeste for posting pictures, videos and comments concerning the royal family on a Facebook account that purported to belong to a different user.
Wichai, whose last name was withheld to protect his relatives from ostracisation, was accused of using the account to slander a former friend, said iLaw, a group that tracks royal defamation cases.
Use of the draconian defamation law has surged under the royalist junta that grabbed power in 2014, with more than 100 people charged since the coup.
Prosecutions have continued under Thailand's new king Maha Vajiralongkorn, who took the throne in late 2016 after the death of his deeply revered father.
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Observers have been closely watching how the new king approaches the controversial legislation, which effectively blocks scrutiny of Thailand's opaque and powerful monarchy.
Reporters were barred from entering the military court where Wichai's verdict was read.
Wichai, who was arrested in northern Chiang Mai in December 2015, initially denied the charges but later confessed after waiting for more than a year in jail for court proceedings to begin, iLaw said.
Lese majeste suspects are rarely acquitted or granted bail.
The UN's human rights office said it was "appalled" by the sentencing and decried the increasing number of lese majeste cases under junta rule.
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