The ruling, which was delivered by Chief judge Judge Dith Munty , who is also a member of the ruling Cambodian People's Party (CPP), barred 118 members of the opposition from politics for five years, reported Al Jazeera.
The CNRP had argued the verdict was pre-determined.
The government accuses the Cambodia National Rescue Party of plotting a coup and has called for its dissolution for weeks.
The opposition has denied the allegations.
The verdict will ensure victory for Prime Minister Hun Sen's ruling party in next year's general election.
According to the reports, Cambodia's Prime Minister, Hun Sen, has ruled for 32 years and has held a tight grip on power since ousting a co-prime minister in a bloody 1997 coup.
A large perimeter surrounding the Supreme Court in the capital Phnom Penh has been sealed off by patrolling police and military.
The decision is the latest in a series of moves against the CNRP.
In September, CNRP leader Kem Sokha was arrested and accused of conspiring with the US to overthrow the government.
He was charged with treason.