In July, Hyundai Motor and Kia Motors sold a combined 70,017 vehicles there, down from 111,021 units a year earlier, the companies' data showed.
The results were mainly affected by political tensions between Seoul and Beijing over the deployment of a US missile defense system here.
In an apparent retaliation against the deployment of the Terminal High Altitude Area Defense system in South Korea, China has taken steps against Korean products.
Beijing has argued the missile defense system could be used against it though Seoul has said it is purely aimed at deterring missile threats from Pyongyang.
In the January-July period, their combined sales plunged 46 percent to 500,964 autos from 919,380 units in the year-ago period, the data said.
Hyundai and Kia together form the world's fifth-biggest carmaker by sales. (Yonhap)