The Court of Appeals upheld the sentence handed to Yosawaris Chuklom, who goes by the stage name Jeng Dokchik, last year in connection with a speech delivered during mass protests against the previous government in 2010.
While he did not explicitly mention the royal family, "the court said my speech led people to understand that I was talking about the monarchy," he told AFP from the court.
King Bhumibol Adulyadej, 86, is revered by many Thais and protected by harsh royal defamation laws.
Critics say the legislation has been politicised, noting that many of those charged are linked to the Red Shirts, who are broadly supportive of fugitive former premier Thaksin Shinawatra.
More From This Section
Royalists have recently urged heightened vigilance against remarks deemed critical of the monarchy.
The call comes against a backdrop of a six-month deadly political crisis that has shaken the government of Prime Minister Yingluck Shinawatra.
Thaksin, her elder brother, clashed with the kingdom's royalist establishment before he was ousted in a military coup in 2006. He lives overseas to avoid prison for a corruption conviction.