An industry group, the China Association of Automobile Manufacturers, says purchases of sedans, SUVs and minivans fell 15 percent from a year earlier to just over 2 million vehicles.
Auto demand has weakened as Chinese economic growth cooled and a tariff fight with Washington fueled uncertainty among buyers.
The slump is a setback for global automakers that are looking to China to drive revenue.
Sales by Chinese brands fell 22 percent while global brands gained market share.
Purchases of electric and hybrid vehicles, which Beijing is promoting with subsidies, rose 138 percent over a year earlier to 96,000 units.