Here we report that the selective trifluoromethylthiolation of unactivated C(sp3)−H bonds is enabled by photoexcited diaryl ketones. This method for C−H trifluoromethylthiolation features mild conditions and inexpensive organic photocatalysts and relies on 370 nm LED lights. The functional group tolerance is broad and the excellent regioselectivity for branched aliphatic positions enables trifluoromethylthiolation of a variety of substrates, including amino acid and natural product derivatives, demonstrating its potential synthetic application.