Jatuporn Prompan, chairman of the red-shirt group, has been handed a one-year jail term for defaming former prime minister Abhisit Vejjajiva in speeches he made in 2009.
The Supreme Court on Thursday sentenced Jatuporn to one year in prison without suspension and ordered him to publish apologies to Abhisit in newspapers for seven days.
Abhisit had filed a case in June 2009, accusing Jatuporn, a leader of the United Front for Democracy Against Dictatorship (UDD), of defaming him in front of 10,000 people through loud speakers at Wat Phai Kiew Temple in Don Muang in Bangkok on May 10.
Jatuporn had called Abhisit a "tyrant" whose "hands are tainted with blood" who deserved to be sentenced to death for killing people.
The Court of First Instance had rejected the case on December 27, 2012, saying that to comment was tantamount to political retaliation in a democratic system.