"Due to the explosion, one section of a five-storey residential building collapsed" in the city of Yaroslavl north of Moscow, the regional emergencies ministry said in a statement.
"Five flats have been completely destroyed. Seven people died, including a teenager and a five-year-old child," it said. The incident happened around 4:20 am (local time).
Three injured people rushed to hospital while authorities evacuated 131 residents and put them in temporary housing.
A spokeswoman for the emergencies ministry told AFP the reason for the blast was not yet clear but Russia's Investigative Committee said it had launched a probe into possible safety violations.
Explosions caused by natural gas leaks are frequent in Russia, especially in older Soviet-era buildings.
The last serious gas explosion in February 2012 killed 10 people in the southern city of Astrakhan and destroyed the entire side of a nine-storey building.
Building collapses are also not uncommon, either due to faulty construction or worn-out infrastructure.
In July, 23 Russian conscripts were crushed to death when their military barracks collapsed in Siberia after renovation works just two years before.
The teacher managed to rush her class out of the room in time, but the entire school building for 400 children is now closed.
