The court ruling also bans 118 Cambodia National Rescue Party (CNRP) members from politics for five years over accusations of plotting to topple the government.
Cambodia's Supreme Court has dissolved the main opposition Cambodia National Rescue Party (CNRP) on Thursday, giving Prime Minister Hun Sen's ruling party a clear run in next year's general election.
The court ruling, in response to a petition brought by the government, also banned 118 CNRP members from politics for five years.
The CNRP, which had been poised to challenge Hun Sen's long rule in the coming election, was accused of plotting to topple the government after the arrest of party leader Kem Sokha on September 3.
CNRP denies accusations
The party has denied the accusations as politically motivated, and did not send lawyers to Thursday's court session.
The verdict comes amid an increasingly tense political situation and a campaign by Hun Sen's ruling Cambodian People's Party (CPP) to crush the opposition ahead of the vote.
More than half the CNRP's members of parliament have already fled Cambodia, fearing detention.
The ruling leaves "no credible political opposition in Cambodia" for the first time since a UN-run election in 1993, a senior diplomat based in the Cambodian capital said.
Dozens of police officers guarded the court, but there were no immediate signs of any protests following the ruling.
Hun Sen has said the CNRP's parliamentary seats will be redistributed to other government-aligned parties after its dissolution.
The judge who heads the Supreme Court, Dith Munty, is a member of the permanent committee of the ruling party and a longtime Hun Sen loyalist.
"The misuse of the courts to dissolve the CNRP is one of the gravest threats to human rights and representative democracy modern Cambodia has seen," said Kingsley Abbot of the Geneva-based International Commission of Jurists.
"(That) should be acknowledged as such by the international community when it sits down to consider its political and economic engagement with the country," he said.
The 2018 election was shaping up to possibly be the biggest challenge to Hun Sen's leadership after his opponents united behind the CNRP, after having made significant gains in local elections in June.