The 55-year-old Thai-born American, Joe Gordon, stood calmly with his ankles shackled in an orange prison uniform as the sentence was read out at a Bangkok criminal court.
Judge Tawan Rodcharoen said the punishment initially was set at five years behind bars, but he reduced it because Gordon pleaded guilty in October.
Gordon posted links the to banned biography of King Bhumibol Adulyadej several years ago while living in the U.S. state of Colorado, and the case has raised questions about the applicability of Thai law to acts committed by foreigners outside Thailand.
Speaking after the verdict, Gordon said: "I am an American citizen, and what happened was in America."
He also said he had no expectation of being let off easy. "This is just the system in Thailand," he said. Speaking later in Thai, he added: "In Thailand, they put people in prison even if they don't have proof."
Gordon was detained in late May during a visit to Thailand, where he had returned for medical treatment. After being repeatedly denied bail, he pleaded guilty in October in hopes of obtaining a lenient sentence.
Thailand's so-called lese majeste laws are the harshest in the world. They mandate that people found guilty of defaming the monarchy - including the king, the queen and the heir to the throne - face three to 15 years behind bars. The nation's 2007 Computer Crimes Act also contains provisions that have enabled prosecutors to increase lese majeste sentences.
The country has come under increasing pressure both at home and abroad to amend the laws, which critics say are too harsh and have often been used for political harassment.
Associated Press writer Todd Pitman contributed to this report.