Temperatures have soared to 38C and are set to reach 40C towards the end of the week. The heat is ruining crops.
It is the second heatwave to hit Romania this year. The previous one in June killed more than 20 people.
The worst-hit areas are rural counties in the south and east. The authorities have sent emergency water supplies from the cities.
Few villagers can afford insurance and government compensation schemes cannot cover their losses.
The authorities have also ordered train drivers to reduce their speed by 20-30 km/h (12-20 mph), to avoid derailments.
At least one person's death has been attributed to the heat.