Vadim Shishimarin, a 21 year old sergeant, was found guilty of killing 62 year old Oleksandr Shelipov in Sumy region during the first days of the invasion.
Shishimarin, who was wearing a blue and grey hooded sweatshirt, watched proceedings silently from a reinforced glass box in the courtroom this morning.
The verdict was delivered on Monday by the judge, Serhii Ahafonov, at a packed courtroom, with dozens of Ukrainian and foreign television cameras crammed into the small room.
Prosecutors said Shishimarin was in a car with other Russian soldiers, one of whom ordered him to shoot Shelipov, as he had been a witness to them shooting at a car and then stealing it.
The judge said although Shishimarin cooperated with the investigation and expressed remorse, the court could not accept his claim he had not meant to kill Shelipov when he fired at him.
He showed no emotion as the verdict was read out.
Ukrainian state prosecutors said the soldier was among four other Russian servicemen who fired at and stole a privately owned car in order to escape after their column was targeted by Ukrainian forces.