The fire, which has since been extinguished, broke out at around 0300 GMT in one of the buildings of the Matei Bals hospital.
The hospital building where the blaze broke out was built in 1953 and had been completely renovated, its manager said.
Matei Bals is one of the largest and most used COVID-19 hospitals in the country.
The evacuated patients had medium to serious COVID-19 infections and most were using oxygen, the hospital's manager said. About 44 of them have been sent to other COVID-19 hospitals across Bucharest, and the remaining patients were re-located to other buildings at Matei Bals.