The conviction is the latest jail term handed down in an unprecedented lese majeste crackdown launched since arch-royalist generals seized power in a May 2014 coup.
Convictions have sky rocketed since the takeover with record breaking jail sentences, many for social media posts, as authorities broaden their interpretation of the law.
Bangkok Criminal Court said 46-year-old Piya Julkittiphan was convicted for posting two pictures with messages in 2013 that risked making the public "disrespectful or unfaithful" to the monarchy. "The judge sentenced him to nine years but he has given useful testimony during the investigation so the court commuted one third of that sentence to six years imprisonment," the court said in its verdict.
The court did not provide details on the content of the posts, as is commonplace in lese majeste convictions.
Even when details are known, media must heavily self censor when reporting on such cases to avoid falling foul of the same law.
Thai Lawyers for Human Rights described Piya as a former stockbroker who was first arrested in December 2014 and has been in custody ever since.
King Bhumibol Adulyadej, the world's longest-serving monarch, is shielded from any debate and criticism by one of the world's harshest royal defamation laws. Anyone convicted of insulting the king, queen, heir or regent, can face up to 15 years in jail on each count.