A lower court found Samak guilty last year. He was sentenced to a two-year jail term for offending former deputy Bangkok governor Samart Ratchapholsit in 2006.
The defamation was found to have been committed on a Channel 9 television talk show and involved bribes offered by a contractor.
He appealed against his conviction. Should the ruling be unfavourable, he is entitled to a final review by the Supreme Court.
His earlier sentence has no bearing on his ability to carry out his job as PM.
He will be disqualified from office if convicted of a felony or if he has to serve time at the completion of the appellate process.