Oliver Rudolf Jufer, 57, was caught by surveillance cameras on Dec. 5 spray-painting black paint over several portraits of King Bhumibol Adulyadej in the northern city of Chiang Mai, police said. His lawyer said he was intoxicated during the act.
The vandalism coincided with Bhumibol's 79th birthday, which was celebrated across Thailand with fireworks and prayers. The king is the world's longest serving monarch.
Jufer made no comment as he entered the courthouse with his legs chained, dressed in an orange prison uniform.
Judge Chaikrit Devaplin said Jufer pleaded guilty, reversing an initial statement of non-guilt that he had made to police, and a sentence was expected to be issued March 29.
The trial was closed to the public, and prosecutors declined to discuss details of the case because of sensitivities in Thailand about portraying the beloved king in a negative light.
"Revealing the details of this case does not benefit anybody because it involves the king and the monarchy," said prosecutor Bhanu Kwanyuen, adding only that Jufer is accused of defacing five posters and faces a penalty of between three and 15 years in prison for each one.
"In every Thai constitution, the king is revered and worshipped, and he cannot be insulted," Bhanu said. "Thai people cannot accept this act of insulting the king."
Jufer has lived in Thailand for more than 10 years, and lives in Chiang Mai, said his court-appointed lawyer, Komkhit Kunyodying.
The military launched a separate investigation into the incident, saying Jufer's act raised suspicion that he was hired by someone opposed to the Sept. 19 coup that ousted Prime Minister Thaksin Shinawatra.
Chiang Mai, about 350 miles north of Bangkok, is Thaksin's hometown and a key base of support for the former prime minister.
Millions of portraits of the king were erected late last year to honor his birthday. Across the kingdom, people also wore yellow shirts, a sign of devotion to the king.
By Ambika Ahuja