Rainsy has been asked to pay almost $40,000 to the state and the plaintiff, National Assembly President Heng Samrin, following the opposition leader posting a video on social networks in November 2015, Efe news reported.
The video showed a speech dating back to 1980 when then monarch Norodom Sihanouk criticised the government, which was then led by Heng Samrin.
Rainsy has been in self-exile in France since last November, after a court issued an arrest warrant to carry out his 2011 conviction in absentia.
Kem Sokha, the vice president of Rainsy's Cambodia National Rescue Party, is also under investigation by the Anti-Corruption Unit. He has been holed up at his party's headquarters since the end of April, when the authorities tried to arrest him for failing to respond to court notices regarding a trial against him.
Twenty-six people continue to languish in Cambodian prisons on charges related to their political activism laid against them since July 2015.
Tensions in the country have heightened since the July 10 murder of political analyst Kem Ley, whose funeral was attended by tens of thousands of people.
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