The case revolved around Chirmsak’s remarks portraying Apipol Kongchanakul, then adviser to Senate speaker Sanit Worapanya, as involved in demanding bribes for job appointments for telecom regulators.
Apipol died in 2005 and his widow, as his estate manager, sued Chirmsak last year.
The court found that Chirmsak had tarnished Apipol’s reputation by not presenting factual evidence linking him to the kickbacks.
As part of his sentence, Chirmsak will still have to pay a fine of Bt100,000 and publish the verdict in two newspapers for two days.