The verdict, handed down by a military court yesterday, comes less than two weeks after the palace sacked the king's deputy chief of staff Chitpong Thongkum for behaving "in a way that brought about distrust and caused severe damage to the royal family".
Palace affairs are often difficult to penetrate in Thailand, where harsh defamation laws shield the royal family from scrutiny and make any discussion of the monarchy extremely sensitive.
In addition to violating the royal insult law, Chitpong was also convicted of "committing theft in state offices" and breaching medical and cosmetic laws, according to a police statement.
"The total prison term is four years and 18 months," the statement said, without elaborating on how the statutes were violated.
Also Read
A military court spokesman declined to comment on the case.
The initial palace statement firing Chitpong accused him of making "false claims" about Vajiralongkorn for personal gain, "releasing the king's personal medical records and stealing his assets".
It said Chitpong was in charge of security for the monarch, who took the throne last year following the October death of his revered father.
Chitpong, a former Air Vice Marshal, is the latest in a string of people close to the new monarch to publicly fall from grace.
On Thursday security officers escorted another senior palace aide, former Grand Chamberlain Jumpol Manmai, to a Bangkok police station to hear charges of land encroachment for building a mansion on park land.
Speculation about Jumpol's fate had swirled for weeks with authorities refusing to disclose his whereabouts after word leaked he was under investigation.
Disclaimer: No Business Standard Journalist was involved in creation of this content