Yuthapoom's wife, Jongkon Kongthin, welcomed the verdict but called the time apart from her husband "torture".
"We had to live without happiness," she said, tearing up. "The court has mercy and this has proven that justice does prevail."
Article 112 of Thailand's criminal code 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". The punishment is one of the world's strictest for such a crime.
Defense lawyer Saovalux Pongam, who took on the case pro-bono, said that "despite this case's dismissal, the core problem of the 112 law remains".
"Anyone can sue anyone and police have to pursue the case," she said. "It's so easy to accuse anyone of this crime."
The judge who read out the verdict cited several reasons for Yuthapoom's acquittal, including the lack of other witnesses besides the plaintiff, Thanawat Martnok. He also cited conflicting accounts that Thanawat gave to police and the court and a history of conflict between the two brothers.
During the trial, the siblings' mother testified that she had never heard Yuthapoom defame the crown and didn't believe he could do so, saying he loved the king dearly and often lit candles at home to commemorate his birthday.
Thanawat had filed the lese majeste complaint well after the alleged transgressions occurred and one month after he moved out of a Bangkok house they shared, fuelling speculation he had done so more out of spite than a desire to protect the crown.
Both brothers fought frequently, arguing about business, politics, and even their dogs.
Lese majeste cases were once rare in Thailand, but they have become more common since a 2006 military coup intensified a bitter societal divide.
The king, who ascended to the throne in 1946 and is now the world's longest-serving monarch, is 85 and ailing but remains widely beloved in the nation of 66 million. Thailand is a constitutional monarchy, and its government is run by an elected parliament and prime minister.
- AP