It borders Ryongrim and Chonchon to the north, the counties of Taehung and Yongwon in South Pyongan to the south and east, and Huichon and Songwon to the west and southwest.

The valley of the Chongchon River lies in western Tongsin.

13% of Tongsin's land is arable, and most of the remaining 87% is forested.

There are no rail lines in the county, but roads tie it to nearby Huichon and Ryonggang, with a bus service operating between Tongsin and Huichon.

Tongsin County is divided into 1 ŭp (town) and 14 ri (villages):