Vadim Shishimarin, a 21-year-old tank commander, had pleaded guilty to killing 62-year-old Oleksandr Shelipov in the northeastern Ukrainian village of Chupakhivka on February 28 after being ordered to shoot him.
Judge Serhiy Agafonov on Monday said Shishimarin, carrying out a "criminal order" by a soldier of higher rank, had fired several shots at the victim's head from an automatic weapon.
"The court has decided: Shishimarin Vadim Evgenyevich ... is found guilty ... and sentenced him to life imprisonment," he said.
"Given that the crime committed is a crime against peace, security, humanity and the international legal order ... the court does not see the possibility of imposing a (shorter) sentence of imprisonment on Shishimarin for a certain period."
Shishimarin, wearing a blue and grey hooded sweatshirt, watched proceedings silently from a reinforced glass box in the courtroom and showed no emotion as the verdict was read out.
For much of the time, he stood with head bowed as he listened to a translator who stood with two guards outside the reinforced glass box.
The trial has huge symbolic significance for Ukraine, which has accused Russia of atrocities and brutality against civilians during the invasion and said it has identified more than 10,000 possible war crimes.
Russia has denied targeting civilians or involvement in war crimes.
The Kremlin did not immediately comment on the verdict. It has previously said that it has no information about the trial and that the absence of a diplomatic mission in Ukraine limits its ability to provide assistance.
Ukrainian state prosecutors said Shishimarin and four other Russian servicemen stole a privately owned car to escape after their column was targeted by Ukrainian forces. The soldiers then drove into the village of Chupakhivka where they saw Shelipov riding a bicycle and talking on his phone, they said.
The prosecutors said Shishimarin was ordered by another serviceman to kill the civilian to prevent him reporting on the Russians' presence and he fired several shots through the open window of the car with an assault rifle at Shelipov's head. Shelipov died on the spot.
In court last week, Shishimarin acknowledged he was to blame and asked the victim's widow, Kateryna Shelipova, to forgive him.
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