The Supreme Court has halved two prison terms given to red-shirt leader Jatuporn Prompan for accusing former prime minister Abhisit Vejjajiva of ordering soldiers to kill protesters.
The court refused to suspend the sentences.
The Supreme Court's sentence was read at the Criminal Court in Bangkok on Thursday. It concerned rally speeches made by Jatuporn, then leader of the United Front for Democracy against Dictatorship (UDD), in 2009 making accusations against Mr Abhisit. He was found guilty on two counts.
Earlier, the Criminal Court and the Appeal Court sentenced him to one year in jail on each count, a consecutive prison term of two years.
The Supreme Court halved the jail term to six months on each count as Jatuporn confessed during the Supreme Court hearing to making the speeches, or 12 months in total.
The Supreme Court rejected Jatuporn's request for suspended sentences, saying his speeches had affected the royal institution.
Mr Abhisit sued the red-shirt leader for wrongfully accusing him of issuing an order for soldiers to kill people and of stalling the submission of a petition seeking a royal pardon for ousted prime minister Thaksin Shinawatra.
Mr Jatuporn made the allegations during UDD demonstrations on Oct 11 and 17 in 2009 when Mr Abhisit, the Democrat Party leader, served as prime minister.
On Thursday officials took Jatuporn from the Bangkok Remand Prison to the Criminal Court. He was earlier sentenced to one year in jail in another defamation case brought by Mr Abhisit.