In this study, we present the preparation of stable 1T-WS₂ ultrathin nanosheets with N₄⁺ intercalation using a bottom-up hydrothermal method and the potential application of this material in light-induced photothermal cancer therapy. Our results revealed that nanosheets with a size of 150 nm were highly hydrophilic and exhibited strong light absorption and excellent photostability in the broad near-infrared wavelength region. The in vitro experimental results indicated good biocompatibility of the nanosheets. More notably, our in vivo antitumor experiments illustrated that light-induced photothermal ablation originating from irradiation of the 1T-WS₂ nanosheets with an 808 nm laser could efficiently kill tumor cells; these effects were obtained not only at the cellular level but also in the living organs of mice. This result may lead to new applications of two-dimensional layered materials in novel photothermal therapies and other photothermal related fields.