SHANGHAI—Ford Motor Co. sales in China plunged 26% in the first half of 2018 compared with the same period last year, and there is little relief in sight as U.S.-China trade tensions worsen.

China’s auto market overall grew 5.6% in the first six months, with sales reaching 14.1 million vehicles, according to figures released Wednesday by the government-backed China Association of Automobile Manufacturers.

But...