The most recent high-profile cases include a probe into the US ambassador for comments he made about jail terms under the lese majeste law and charges against a man for allegedly insulting the king's dog.
Chayapa Shokpornbudsri, 49, was found guilty of defaming the monarchy on Facebook at a hearing without her lawyer present, a court official said today.
She was handed a 19-year sentence which was cut in half after she pleaded guilty, the official said, adding it was "a secret ruling".
Thailand has one of the world's harshest lese majeste laws and prosecutions have soared since the army, which styles itself as the champion of the monarchy, grabbed power in a coup last year.
Any member of the public can make an allegation of royal defamation, which the police are duty bound to investigate.
Critics say that situation often results in effective witch-hunts, led by ultra-nationalist volunteers who comb through social media and public events for possible breaches of the law.
Chayapa's lawyer Khumklao Songsomboon, from Thai Lawyers for Human Rights, told AFP today that she had not been informed of the court hearing in advance, as was customary.
Chayapa "was crying when she rang yesterday... I only knew about the verdict after it had happened," Khumklao said.
"I don't know why (they didn't tell me). I don't know what the court's intention was," she said, adding it was a violation of her client's human rights.