Sgt Vadim Shishimarin was accused of shooting the civilian in the head in a village in the north-eastern Sumy region in the early days of the conflict, which started on February 24.
He told the court an officer insisted the victim, who was speaking on his mobile phone, could pinpoint their location to Ukrainian forces.
During the trial, Shishimarin asked the man’s widow to forgive him.
Shishimarin’s defence lawyer Victor Ovsyanikov argued that his client, a member of a Russian tank unit who was eventually captured, had been unprepared for the “violent military confrontation” and mass casualties that Russian troops encountered when they first invaded Ukraine.
The sentencing came as the three-month-old war helped push the number of people displaced worldwide to a record high, according to the United Nations, with more 100 million people driven from their homes across the globe.
Meanwhile, Ukrainian President Volodymyr Zelensky addressed the World Economic Forum as it opened in Davos, Switzerland, calling for “maximum” sanctions against Russia.
He said by video that sanctions needed to go further to stop Russia’s aggression, including an oil embargo, all of its banks blocked and cutting off trade with Russia completely.
Mr Zelensky said his country has slowed Russian advances and his people’s courage has stirred unseen unity of the democratic world.
On the battlefield, Russian forces have stepped up shelling in Ukraine’s eastern industrial heartland as they press their offensive in the region that is now the focus of fighting.
Grinding battles in the Donbas, where Ukrainian and Russian forces are fighting town by town, have forced many civilians to flee their homes.
In Tokyo on Monday, US President Joe Biden and Japanese Prime Minister Fumio Kishida joined in condemning Moscow’s invasion of Ukraine.
Earlier on his trip to Asia, Mr Biden signed legislation granting Ukraine 40 billion dollars (£31.8 billion) more in US support for its defence against the Russian attack.
Western support - both financial and military - has been key to Ukraine’s defence, helping their outgunned and outnumbered forces to repel Russia’s attempt to take the capital of Kyiv and fight them to a standstill in other places.
In the face of those setbacks, Moscow has outlined more limited goals in Ukraine, with its sights now on trying to expand the territory that Russia-backed separatists have held since 2014.
Ukrainian forces dug in around Sievierodonetsk, the main city under Ukrainian control in the Luhansk province of the Donbas, as Russia intensified efforts to capture it.
Governor Serhiy Haidai accused the Russians of “simply intentionally trying to destroy the city ... engaging in a scorched-earth approach”.
Mr Haidai said on Sunday that the Russians had occupied several towns and cities in Luhansk after indiscriminate, 24-hour shelling and concentrating forces and weaponry there, bringing in troops from Kharkiv to the north-west, Mariupol to the south, and from inside Russia.
But the Ukrainian military said that Russian forces were unsuccessful in their attack on Oleksandrivka, a village outside of Sievierodonetsk.
Ukraine’s parliament voted on Sunday to extend martial law and mobilise its armed forces for a third time, until August 23.
Ukrainian officials have said little since the war began about the extent of their country’s casualties, but Mr Zelensky said on Sunday that 50 to 100 Ukrainian fighters were being killed, apparently each day, in the east.
While the east is now the focus of flighting, the conflict is not confined there.
Powerful explosions were heard early on Monday in Korosten, about 100 miles west of Kyiv, the town’s deputy mayor said.
It was the third straight day of apparent attacks in the Zhytomyr District, Ukrainian news agencies reported.