Investigators said Hubbard, a native of Michigan, was paid $US1000 ($A1500) a month to serve in a Ukrainian territorial defence unit in the eastern city of Izyum, where he had been living since 2014.
They said Hubbard was provided with training, weapons and ammunition when he allegedly signed up in February 2022, the same month Moscow sent thousands of troops into Ukraine.
He was detained by Russian soldiers on April 2 of that year, the RIA state news agency quoted the prosecutor as saying last month.
Russian state media said Hubbard had pleaded guilty to the charge.
But in interviews in September, Hubbard's sister Patricia Hubbard Fox and another relative cast doubt on his reported confession, telling Reuters he held pro-Russian views and was unlikely to have taken up arms at his age.
On Monday, Hubbard, wearing a beige sweater, sat in a glass courtroom cage in handcuffs.
He stood up, seemingly with difficulty, to hear the judge in the Moscow City Court pronounce him guilty, removing his hat to reveal a shaved head.
Hubbard listened without visible emotion to the judge before conferring with his lawyer, who later declined to comment to reporters.
Though RIA reported that Hubbard's lawyer will appeal the verdict.
Reuters was unable to confirm how Hubbard was detained.
The Ukrainian foreign ministry has not replied to multiple messages seeking comment.
A spokesperson for the US embassy in Moscow said it was aware of the detention of an American citizen, but declined further comment on Monday.
In interviews, Fox and the other relative portrayed Hubbard as an isolated figure who had grown estranged from some of his family during his decades abroad teaching English, including in Japan and Cyprus.
Fox said Hubbard moved to Ukraine in 2014 and lived there for a time with a Ukrainian woman, surviving off a small pension of around $US300 a month.
He never learned Russian or Ukrainian, and had few connections to locals, she said.
Hubbard is one of at least 10 Americans behind bars in Russia, nearly two months after a prisoner swap on August 1 between Moscow and the West freed three Americans and dozens of others.
Separately, on Monday a court in Voronezh, south of Moscow, sentenced US citizen and ex-marine Robert Gilman to seven years and one month in prison for assaulting a prison official and a state investigator while serving time for an earlier assault conviction.
