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.
But tourism has dipped sharply since gunmen from the so-called Islamic Statekilled 130 people in the November Paris attack.
The city was only just beginning to recover from an attack in January 2015 on the satirical magazine Charlie Hebdo.
Tourist board figures show that nightly hotel stays were down 8.5% in the Ile-de-France region in the first half of 2016, with an 11.5% decline in foreign tourists and a 4.8% decline in French tourists.