Surachai Danwattananusorn, 71, was convicted of making defamatory speeches during rallies organised by the red-shirt political movement in 2010.
It is the latest in a series of cases brought under the lese majeste law.
It protects the most senior members of the royal family from insult, but rights groups say it is being used to suppress legitimate debate.
Mr Surachai was initially sentenced to 15 years, but the court cut this in half after he pleaded guilty to some of the charges.
The well-known activist, who was arrested a year ago, has been very vocal in condemning the 2006 coup that ousted former Prime Minister Thaksin Shinawatra, who the red-shirts broadly support.
Mr Surachai’s lawyer says that the charges were politically motivated.
In recent years, the law has been used far more frequently and far more widely than in the past.
The United States, the European Union and human rights groups have expressed concern about the way in which the law is being applied.