The initial sentence of 15 years, five years each for three offenses, was halved because the 58-year-old actor Thanat Thanawatcharanont pleaded guilty to the charges presented last week, state-run daily Post Today reported.
In November 2013, Thanat gave a speech for which he was later tried while participating in protests in support of the former Prime Minister Yingluck Shinawatra, who was overthrown months later in a coup.
The actor, also known as Tom Dundee, was arrested in July 2014 and has since remained in prison, EFE reported.
Article 112 of the Thai Criminal Code punishes those convicted of insulting, defaming or threatening the king, queen, crown prince or regent with a jail term of between three and 15 years.
Although King Bhumibol himself declared in 2005 that he is not above criticism, the prosecution for this charge has soared in recent years due to political divisions in the country.
After the military coup in May 2014, the number of lese majeste prosecutions has multiplied.
On various occasions, the army has sent suspects to military courts whose sentences are twice as severe as those of civil courts, according to the human rights defence organisation iLaw.