Somyot Pruksakasemsuk was convicted of publishing two articles in an anti-establishment magazine that made negative references to the crown.
The verdict came despite repeated calls by rights groups to free Somyot, who has been jailed since 2011. It also underscored the harsh nature of Thailand's lese majeste laws, which critics say have frequently been abused by politicians' intent on silencing rivals.
The articles in question were published under a pseudonym in Somyot's now-defunct Voice of Taksin magazine, which he launched in 2009 to compile political news and anti-establishment articles from writers and contributors.
Judges found both pieces contained content that defamed the royal family and argued that Somyot, as a veteran editor, knew that and chose to print them anyway. The court announced two five-year jail terms—one for each story.
"(Somyot) should have better judgment than ordinary journalists. He must have understood that the articles contained lese majeste content, but chose to publish them anyway," one of judges said in the sentence.
Somyot said he would appeal the verdict but would not seek a royal pardon.
Brad Adams, Asia director of Human Rights Watch, said
Although the articles were published in 2010, Somyot was only arrested the following year—five days after launching a petition drive to revoke Article 112 of the nation's criminal code, which mandates three to 15 years in jail for "whoever defames, insults or threatens the king, the queen, the heir to the throne or the regent."
Human Rights Watch said the author of the articles, who penned them under pseudonym, has never been charged with any crimes and is living in Cambodia.
The European Union also weighed in on the verdict, saying it "seriously undermines the right to freedom of expression and press freedom" and "affects Thailand's image as a free and democratic society."
More than 100 observers, including Thai and international scholars and journalists, diplomats and Somyot's supporters, were in court to hear the verdict.
Somyot, who was brought in with his legs shackled, joked to a friend that he would no longer need books to read in prison because he thought he would be freed.
In addition to the 10-year punishment, Somyot was also sentenced to a one-year term in a separate criminal case in which he was charged for alleging a Thai general was behind the country's 2006 army coup.
The coup ousted Prime Minister Thaksin Shinawatra and sparked years of sometimes violent political unrest from which the nation has yet to fully recover. Somyot was also a leader of the so-called Red Shirt movement, which supported Thaksin.
"His guilty verdict and sentence should be viewed as a sign that Thailand's deep political schisms are far from healed," Adams said.
Associated Press writer Todd Pitman contributed to this report.