Ultrathin In₄SnS₈ nanosheets have been successfully synthesized via a facile thermal decomposition method. The average thickness of these In₄SnS₈ nanosheets is only 3.8 nm, comprising about five atomically thick layers. To our knowledge, this is the thinnest In₄SnS₈ nanosheet synthesized using a solution-phase chemical method. The resulting ultrathin In₄SnS₈ nanosheets exhibit fast adsorption–visible-light photocatalysis dual function for various organic dyes, suggesting their potential application in environmental remediation, solar energy conversion, and advanced optical/electric nanodevices.