Ampon Tangnoppakul, 61, was convicted of sending four messages last year to an official working for then Prime Minister Abhisit Vejjajiva.
He was charged under the Computer Crimes Act and lese majeste law, which is designed to protect the monarchy.
Critics say both laws have been increasingly politicised and are curbing free speech in Thailand.
Thailand’s criminal code defines lese majeste as defaming, insulting or threatening the king, queen, heir-apparent or regent.
Offenders can receive 15 years in jail for each offence.
In recent years the law has been used far more frequently and far more widely than in the past.
There have been widespread allegations that the law is being misused to settles scores and silence debate rather than protect the monarchy.
Several foreigners have been convicted of the offence in recent years, but they are often quickly pardoned and deported from the country.
Some Thai academics and writers have fled the country for fear of being denounced.