The fire at the home for the elderly near Tula, 200 km (120 miles) south of Moscow, engulfed an area of 1,700 sq meters (18,300 sq feet) and took firefighters five hours to put out. About 300 people, including medical staff, were evacuated and transferred to other homes.
A local administration spokesperson quoted Alexei Korablyov as saying that seven bodies have so far been identified.
The regional prosecutor's office said Monday prosecutors are investigating possible fire safety violations and staff negligence.
The old, two-story building with wooden partitions did not have a fire alarm system fitted, investigators said.
Wednesday has been declared a day of mourning in the region.
It is the latest in a series of deadly blazes, which have hit Russia recently, highlighting problems facing state-run institutions.
In a similar incident in June, at least 10 people were killed and four seriously injured in a fire at a retirement home in West Siberia.
Sixty-one pensioners and medical staff died in a fire at a retirement home in the Krasnodar region in southern Russia in March. Last December, 46 patients at a Moscow drug treatment clinic died in a fire. And at least nine students died and dozens were injured in a blaze at a Moscow dormitory last October.