There were a million fewer visitors between January and June compared with the same period in 2015.
Paris welcomes 16 million visitors a year and is one of the world's top tourist destinations.
The drop is estimated to have cost about €750m (£644m) in lost revenue. One senior official described it as "an industrial disaster".
France relies heavily on tourism, which generates more than 7% of its annual GDP.
About half-a-million people in the Ile-de-France region, which includes Paris, have jobs linked to tourism, making it the biggest employer in the area.