ITV News Correspondent Dan Rivers on the significance of the sentencing
Sergeant Vadim Shishimarin, 21, was convicted of killing 62-year-old Oleksandr Shelypov in the north-eastern Sumy region on February 28, four days into the invasion.
He pleaded guilty and admitted shooting Mr Shelypov in the head through an open car window - but said he had been acting on orders.
During the trial, Shishimarin asked the widow of Mr Shelypov to forgive him.
On Monday, the court in Kyiv sentenced Shishimarin - a captured member of a Russian tank unit - to life behind bars.
It marks the first guilty conviction for war crimes since Moscow launched its invasion three months ago. Russia denies its soldiers have killed civilians.
Shishimarin in the defence box moments after hearing his sentence
Ukraine’s Prosecutor General Iryna Venediktova has previously said her office was preparing war crimes cases against 41 Russian soldiers for offences including bombing civilian infrastructure, killing civilians, rape and looting.
Shishimarin had told the court an officer insisted the Ukrainian man, who was speaking on his mobile phone on the pavement, could pinpoint their location to the Ukrainian forces.
The prosecution had argued that Shishimarin was ordered to kill him so he wouldn’t be able to report them to Ukrainian military authorities.
His lawyer, Victor Ovsyanikov, argued his client had been unprepared for the “violent military confrontation” and mass casualties that Russian troops encountered when they first invaded Ukraine.
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Mr Shelypov's wife, Kateryna Shelypova, had earlier told ITV News that her husband, who served with the KGB during the Soviet era, was killed just outside their home, close to where Russian troops were allegedly trying to enter a neighbour's property.
"They have brought too much grief to us," she said, referring to Russian soldiers, whose actions, she said, are unforgivable.
"Too many children have died. There has been too much brutality."
Speaking about her husband's killer, she had said: "What can I say? Him being a child, he is young I feel sorry for him. We didn’t ask them (the Russians) to come here."
She had told the court that her husband had gone out to check what was happening outside, when gunshots rang close by.
When the shooting ceased shortly after, she walked out and found her husband shot dead just outside their home.
She had told the court that Shishimarin deserved a life sentence for killing her husband but added that she wouldn’t mind if he was exchanged as part of a possible prisoner swap with Russia for the surrendered Ukrainian defenders of the Azovstal steel plant in Mariupol.
Mr Ovsyanikov had urged the court to base its judgement on the circumstances behind him firing the gun and had reminded it that any verdict was "subject to appeal".
He had told the court: "Could any of the people present here understand these circumstances in that situation?”
The prosecution told judges that the defence's arguments "do not deny the guilt of Shishimarin himself in this criminal offence.”