The hospital, located in northwestern Moscow, had been designated by the authorities as one of the medical facilities treating coronavirus patients in the Russian capital.
The emergency ministry told the RIA news agency that the blaze erupted in a patient’s room, without providing further details. The fire has since been put out, the authorities said.
Sergei Sobyanin, the mayor of Moscow, wrote on Twitter that all patients had been evacuated and would be transferred to other hospitals.
“The causes of this incident will be thoroughly investigated,” Sobyanin wrote.
Moscow and several other regions have been on lockdown since late March to stem the spread of the coronavirus, which has so far infected nearly 200,000 people and caused more than 1,800 deaths in Russia.
More than half of the country’s cases and deaths have been recorded in Moscow, a sprawling city of 12.7 million.
The number of Russian coronavirus cases this week overtook French and German infections to become the fifth-highest in the world.