The boy was discovered by rescuers during their search for the dozens of people still missing after the disaster on Monday.
The blast, thought to have been caused by a gas leak, damaged 48 apartments in a 10-storey building in the industrial city of Magnitogorsk.
The infant was diagnosed with a serious freezer burn and a closed-head injury. His mother also survived the blast.
Officials were quoted as saying by Interfax: “Hundreds of people were waiting for the appearance of the injured child from under the rubble like a miracle. And the miracle happened... Tears drew in the eyes of the weatherbeaten rescuers.”
On New Year’s Day the death toll was revised up to nine, with more than 30 still missing.
Five more are in hospital for their injuries, with one of those said to be in a critical condition.
The latest fatality is thought to be an adult.
Officials said their search for survivors was being hampered by concerns that moving rubble could cause additional sections of the building to collapse.
A delay of up to 24 hours was expected in the rescue effort while emergency services shore up parts of the block.
Hopes of finding survivors were further dimmed as overnight temperatures fell to -18C in Magnitogorsk, a city of 400,000 about 870 miles southeast of Moscow.
The explosion damaged 48 apartments, housing around 110 people (EPA)
Emergency officials have deployed powerful heaters to raise temperatures under the debris.
“We must work as quickly as we can as temperatures don’t give us any time to linger,” said Pavel Baryshev, Russia’s deputy emergency minister.
Dozens were safely evacuated from the building after it was partially destroyed on New Year’s Eve.
Vladimir Putin has ordered a special government commission to coordinate the Kremlin’s response to the incident.
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Officials fear many of those unaccounted for have been buried beneath rubble after the doorway to the block collapsed.
Local media reported cries for help coming from the scene and a smell of gas in the area.
Watch moreOf more than 100 apartments in the building, 48 were damaged, the emergencies ministry said.
Fires and explosions from gas leaks are not unusual in Russia, especially in smaller cities.
Soviet housing stock and poor maintenance are often blamed, with dozens of people losing their lives each year.
In 2017 a nine-storey residential block in Izhevsk, central Russia, partially collapsed following a gas explosion.