The Phnom Penh Appeals Court on Thursday also barred Kem Sokha from leaving the country for five years after he finishes his sentence, which he is serving under house arrest.
Cambodia's government has long been accused of using the judicial system to persecute critics and political opponents.
The government insists it promotes the rule of law under an electoral democracy, but political parties seen as potential rivals have been dissolved by the courts or had their leaders jailed or harassed.
Kem Sokha, now 72, was convicted in 2023 following a long pre-trial detention.
He was accused of conspiring with the United States to topple the Cambodian government.
The primary evidence against him was a video of him discussing political advice from US-based pro-democracy groups.
His Cambodia National Rescue Party had been the only credible opponent of the ruling Cambodian People's Party, but it was dissolved by the Supreme Court after his arrest and was unable to contest the 2018 election.
The party of long-serving prime minister Hun Sen subsequently swept all the seats in the National Assembly.
His son, Hun Manet, succeeded him in the position in 2023 but has not liberalised the system.
Speaking before the appeals court earlier in April, Kem Sokha declared he had never conspired with any foreign country at the cost of the lives of Cambodian citizens or the loss of national territory and that he had acted in a spirit of non-violence and national unity.
Kem Sokha's lawyer, Pheng Heng, told reporters he regretted the court's decision and that he and his client would have to decide whether to appeal further to the Supreme Court.
He urged the government to promote national reconciliation.
Western embassies in Cambodia expressed concern over the ruling.
"We would like to see Kem Sokha released and his political freedoms restored," the British embassy said in a statement.
" We believe this would help strengthen democracy in Cambodia."
