Drivers in the world's biggest market by number of vehicles bought 1.3 million cars, minivans and other passenger vehicles, according to the China Association of Automobile Manufacturers.
Sales growth that peaked at 45 percent in 2009 has declined steadily as the world's second-largest economy cooled and cities imposed ownership limits to curb smog and congestion.
The fall in recent months has been far sharper than expected, leading General Motors Co., Volkswagen AG and Chinese SUV brand Great Wall Motors to cut prices.
July's slide in sales followed June's 3.4 percent fall, which was only the third such monthly contraction since September 2012.
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This year's downturn has been so abrupt that industry analysts who expected sales growth of up to 8 percent have cut their forecasts to as little as 1.7 percent.
Total vehicle sales, including trucks and buses, contracted by 7.1 percent in July to 1.5 million vehicles, according to CAAM.
For the seven months through July, passenger vehicle sales rose 3.9 percent over a year earlier to 1.1 million units. Total vehicle sales came close to stalling, rising just 0.4 percent over the same period of 2014 to 1.3 million.
Sales by Chinese brands rose 5.1 percent to 494,000 and their total market share rose by 1.2 percentage points to 38.9 percent.
General Motors Co., which competes with VW for the status of China's biggest-selling auto brand, said sales by the company and its Chinese partners declined 4 percent in July to 229,175 vehicles. For the year to date, sales rose 3.3 percent to 1.9 million.
Nissan Motor Co., the most popular Japanese brand in China, said July sales contracted 13.9 percent to 84,200 vehicles. For the year to date, sales rose 2.8 percent to 672,100 units.
BMW AG, Europe's biggest luxury brand, gave no July figures but said deliveries of BMW and Mini vehicles rose 1.3 percent in the first seven months of the year to 265,215.