The charge was brought against him after he demanded that law enforcement agencies prosecute a woman accused of anti-monarchist activities and disseminated part of her speech at a yellow-shirt rally to support his demand.
The court ruled that Sondhi had no intention of violating the law in his 2008 comments and dismissed the charge.
Media mogul Sondhi was a founding member of the People’s Alliance for Democracy (PAD), commonly known as the yellow-shirt group.
He said the charges against him originated from unspecified political rivals.
"There is an effort to put me in jail," he told reporters, alleging that some elements in the court system had worked in collusion with politicians.
The woman Sondhi quoted, Daranee Charnchoengsilapakul -- a hardcore supporter of the rival red-shirt group -- was jailed for 15 years in December for her comments, which she made during political rallies four years ago.
In 2010, Sondhi was convicted of defaming fugitive former prime minister Thaksin Shinawatra and given a six-month suspended jail sentence.
In February this year, the Criminal Court sentenced him to 20 years in prison for corporate fraud in a case dating back to the mid-1990s. He was released on bail pending an appeal on those charges.