Navalny is already serving two and a half years in a prison about 100km (60 miles) east of Moscow for violating parole on old fraud charges.
As his new sentence comes into force, the 45-year-old will be transferred to a strict regime penal colony with harsher conditions, including fewer family visits.
The new sentence will replace the one he was handed in February 2021, meaning Navalny will remain behind bars for another eight years.
Navalny joined the hearing on Tuesday at a Moscow court via video link from behind bars at his prison colony to appeal against the extension of his current jail term.
Wearing a black prisoner uniform and a fur-collared winter jacket, he appeared composed and joked about problems with the sound system.
The hearings started last week but Navalny was granted a week’s postponement so he could have a family visit.
The hearings come as Russian authorities seek to silence remaining government critics and Moscow pushes on with its military campaign in neighbouring Ukraine.
A vocal critic of Russian President Vladimir Putin, Navalny in late March had his jail time extended to nine years after he was found guilty of embezzling donations to his political organisations and contempt of court.
His lawyer Olga Mikhailova told the court the sentence should be annulled as it is “unjust” and “contradicts international law”, while the prosecutor called it “legal and justified”.
Navalny told the court his case was heard in prison on the pretext of COVID restrictions although the relevant Moscow court was functioning.
“The COVID restrictions only applied to me,” he said.
He also alleged his legal team “caught judge (Margarita) Kotova right in the middle of the trial calling someone from the presidential administration”.
Navalny rose to prominence as an anti-corruption blogger and, before his imprisonment, mobilised anti-government protests across Russia.
In 2020, he narrowly survived a poisoning attack with Novichok, a Soviet-designed military-grade nerve agent. Despite accusations from Navalny, the Kremlin denied any involvement.
He was arrested last year on his return from treatment in Germany, sparking widespread condemnation abroad and sanctions from Western capitals.
Navalny’s political organisations across the country have since been declared “extremist” and shut down.
His key allies have fled Russia and several are wanted by Russian authorities on criminal charges.