The capital has lost about 750m euros (£644m) in revenue since January due to the fall in visitor numbers, officials said.
And the number of nights booked in hotels is down 11.4%.
Strikes and floods have also played a part, with tourism across the whole of France down by 7%.
Wealthier tourists in particular were staying away from Paris, with high-end places to stay reporting declines of between 30-40%.
The number of Japanese visitors almost halved in the first half of the year, compared to the previous year, according to tourist board figures.
Russian numbers were down by more than a third, Italians by a quarter and Chinese visitors by almost a fifth.
The industry is going through an “industrial disaster” and is now in need of massive new investment and a rescue package, said Frederic Valletoux, head of the Paris region tourist board.
Visitor numbers dropped following the terror attack in Paris last November, when 130 people were killed.
Officials had hoped the trend would reverse following the Euro 2016 football championships.
But just days after the competition ended, a man believed to be acting for Islamic State drove a lorry into a crowd celebrating Bastille Day in Nice, killing 86 people.
Two weeks later, two men suspected of belonging to the so called Islamic State group killed a priest in a small town in Normandy.
“The Nice attack derailed our hopes of a recovery. It’s a dramatic situation and there will be job cuts in the sector if things do not get better by the end of the year,” said Christian Navet, head of the UMIH-Paris-Ile-de-France hotel federation.
Hotel revenues were down 15% this summer in the Paris region, said Georges Panayotis, head of hotel research firm MKG group.
Only massive new investments can help protect jobs, said the tourist board’s Valletoux, urging foreign minister Jean-Marc Ayrault to meet officials from the sector.
Mr Ayrault said he was adding 500,000 euros to the 1m-euro budget the state was contributing, under a global 2.5m-euro plan unveiled in March to promote France as a tourist destination.