The Phnom Penh Municipal Court fined Rainsy for alleging that National Assembly President Heng Samrin called former King Norodom Sihanouk a "traitor" in 1979 and sentenced him to death in absentia under a Vietnamese-backed government.
Rainsy made the claims in a Facebook video published in November 2015.
Rainsy has been in self-imposed exile in France since November 2015, when the same court issued an arrest warrant for him based on a seven- year-old criminal defamation charge.
The entire trial on Thursday took 10 minutes, according to local rights group, which monitors the political climate, although it did not include any testimony from Rainsy or his lawyer.
The conviction is the latest in a series of legal and physical attacks on Cambodia's opposition, most notably the death of politician and analyst Kem Ley, who was shot in public last month in what was widely seen as a politically 1motivated killing.
- DPA