The ruling came just a few days after ousted Prime Minister Yingluck Shinawatra, whose government was backed by the red shirts before last year's military coup in May, was banned from politics for five years.
The Criminal Court found Jatuporn Prompan, chairman of the United Front for Democracy Against Dictatorship (UDD), or red shirt group, guilty of defaming former Prime Minister Abhisit Vejjajiva in two speeches in October 2009.
The court upheld a lawsuit filed by Abhisit who said Jatuporn, a former Pheu Thai party member of parliament, made defamatory remarks about him in his speeches.
In his speeches, Jatuporn had accused Abhisit of ordering the killing of the people and obstructing the submission of a petition to the King seeking a royal pardon for former prime minister Thaksin Shinawatra, Bangkok Post said.
Abhisit's government had faced major protests in April 2009. The military's crackdowns on protesters had claimed the lives of many people.
His lawyer said he would apply for bail for his client.
