Lev Shlosberg, the party's deputy chairman, was accused of discrediting Russia's armed forces and spreading false information about them.
In a closing speech to the court in the city of Pskov last Friday, he maintained his innocence, said he was the victim of a political trial and repeated his call for an immediate ceasefire in the four-and-a-half-year war.
The verdict, published by the court, follows an August 10 decision by Russia's Supreme Court to exclude Yabloko from taking part in a parliamentary election next month.
The Supreme Court on Monday rejected an appeal against that ban.
Yabloko said it would now lodge a supervisory appeal with the Presidium of the Supreme Court.
Party leader Nikolai Rybakov had called on the court to reinstate Yabloko's registration for the September 18-20 election.
Yabloko, the only registered political party to call for an end to the war, has come under heavy pressure from the authorities in the run-up to the election, where any sizeable anti-war vote could have embarrassed the Kremlin.
Shlosberg used his closing speech to deliver a searing indictment of the war.
"The lives of countless thousands of people have been cut short during this time. Sooner or later, the precise and complete death toll - with names included - will be made public, and the entire nation will shudder at this list of martyrs," he said, according to a transcript by independent news outlet Mediazona.
"Today, cemeteries in our country, including military ones, are growing far more rapidly than maternity wards, kindergartens and schools," he said.
Mediazona and the BBC Russian service have recorded the names of at least 239,354 Russian military dead, based on publicly available records.
Based on records of the estates of dead people, which they used to calculate excess mortality among Russian men, they estimate the true toll at more than 350,000.
Neither Russia nor Ukraine publishes up-to-date casualty figures.
Russia says it is fighting to address security threats from the NATO military alliance and Ukraine, and to defend the rights of Russian-speakers living in Ukraine.
Ukraine and its allies reject these as a false premise for launching the deadliest conflict in Europe since World War II.
Russia passed laws soon after its full-scale invasion of Ukraine in February 2022 that prescribed long prison terms for those found guilty of "discrediting" the military or deliberately spreading false information about the war.
As a result, public condemnation of it is rarely heard inside the country.
But individuals such as Shlosberg, human rights campaigner Oleg Orlov and the late dissident Alexei Navalny, defending themselves at trials with no realistic chance of acquittal, have seized the opportunity to speak out from the dock against what they have described as tyranny and repression.
Such criticism is not reported by state media but can be read online by those with access to virtual private networks that allow them to bypass state-imposed internet restrictions and get access to websites that would otherwise be blocked.
Mediazona said Shlosberg's speech was repeatedly interrupted by the judge, who urged him to stick to the facts of the case against him.
But he insisted on his right to continue.
"A single person is imprisoned, yet fear seeps into the hearts of millions," he said.
The politician was first detained in June last year and charged with discrediting the army after describing the war in Ukraine as a game of "bloody chess" in a video debate.
with DPA
