The incident occurred overnight near the top ski resort of Cortina d'Ampezzo in the Dolomites range in the northeast of the country.
The car was recovered in a river bed covered in detritus but emergency services were unable to resuscitate the woman.
The tragedy follows the deaths on Thursday of two pensioners, a 79-year-old woman and an 82-year-old man, who were caught up in wild fires in, respectively, the central region of Abruzzo and near Matera in the south of the country.
Fires have been breaking out across Italy as a result of a heatwave which has seen peak daytime temperatures top 40°C (104°F) across most of the country for over a week.
Humidity and other factors are making it feel much hotter with the so-called 'perceived' temperature in Campania, the region around Naples, estimated at a broiling 55°C (131°F) on Friday.
Hospital admissions are running 15-20 percent above seasonal norms and food producers are forecast to suffer billions of dollars in losses as a result of reduced crop yields.