Kem Sokha was arrested in 2017 and his opposition Cambodia National Rescue Party (CNRP) was banned before a 2018 election that was swept by the Cambodian People's Party (CPP) of Prime Minister Hun Sen.
CNRP has since been decimated, with many of its members arrested or fleeing into exile in what activists say is a sweeping crackdown designed to thwart challenges to CPP's power monopoly.
"I hope ... the court will decide to drop the charges against me so that we can reach national reconciliation and national unity to develop our country," Kem Sokha told reporters from his home before arriving at the Phnom Penh Municipal Court.
Kem Sokha was freed from house arrest in 2019 but remains banned from political activities.
His daughter Monovithya Kem on Tuesday had also urged the court to drop the charges, adding that her father was in "strong spirits".
The treason charges stem from accusations he was conspiring with the United States to overthrow self-styled strongman Hun Sen, who has ruled Cambodia for nearly four decades.
Kem Sokha denies the charges and the United States has dismissed the allegations as "fabricated conspiracy theories".
The US embassy in Phnom Penh urged authorities to stop "politically motivated trials", including that of Kem Sokha and other members of the political opposition, journalists, and labour and environmental activists.
Cambodia's justice ministry has said the trials were not politically motivated and urged the US embassy to provide evidence to support its claim and not to interfere.
