More than 100 party members are now banned from politics for five years.
Cambodia's Prime Minister, Hun Sen, has ruled for 32 years.
He has long been accused of using the courts and security forces to intimidate opponents and crush dissent, but has for years allowed some measure of political opposition to his Cambodian People's Party.
The CNRP made unexpectedly strong gains in the 2013 elections, and had been set to fiercely contest next year's polls.
Senior CNRP politician Mu Sochua, who has fled the country along with dozens of other MPs, told the BBC that the decision marked "the end of true democracy in Cambodia".
She called for sanctions, adding: "The international community cannot let democracy die in Cambodia by refusing to see that its has been dealing with a dictator for the past three decades."
The president of the Supreme Court - Judge Dith Munty - is a member of the ruling party.
In announcing his decision he said that the CNRP had effectively confessed to the charges of plotting a revolution by not sending any lawyers to the trial.
The CNRP had argued the verdict was pre-determined.
During the hearing, a government lawyer said the opposition had tried to topple the government "in order to grab power, like in Yugoslavia, Serbia and Tunisia", the Phnom Penh Post newspaper reported.
Senior government figures have long warned of a US-backed "colour revolution" in Cambodia, alleging links between independent NGOs, US-linked media outlets and the CNRP.
Prime Minister Hun Sen had earlier called on CNRP lawmakers to defect to his own party ahead of the ruling. He also said he was sure the party would be dissolved, adding: "I dare to bet my life on this happening".
There was tight security outside the court building in the capital Phnom Penh on Thursday, with Cambodian riot police standing guard.
The ruling comes after a prolonged crackdown on critics and dissent. In September, CNRP leader Kem Sokha was arrested and accused of conspiring with the US to overthrow the government. He was charged with treason.
Several US-backed media outlets and organisations have also recently been shut down or kicked out of the country.
In September, one of Cambodia's last independent newspapers, the Cambodia Daily, was forced to close after the government ordered it to pay a huge tax bill.
Hun Sen, a former Khmer Rouge commander, is one of the longest-serving heads of government in the world.
He has overseen a prolonged period of peace and rapid economic growth in the war-scarred country but is criticised by groups like Human Rights Watch for being a ruthless leader unwilling to relinquish his hold on power.
In recent years he has positioned his government closer to China, which has showered Cambodia with aid and loans.
In 2015, then CNRP leader Sam Rainsy fled to France to escape arrest for a defamation conviction. He quit the CNRP in February, but said on Wednesday that he would return to politics.
Dozens of CNRP MPs also left the country ahead of the Supreme Court ruling, fearing arrest.
Earlier this year, the party made further gains in local elections that were seen as a bellwether for next July's polls.
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