Theranostic nanoagents that integrate the diagnoses and therapies within a single nanomaterial are compelling in their use for highly precise and efficient antitumor treatments. Herein, polyethylene glycol (PEG)-modified cobalt sulfide nanosheets (CoS-PEG NSs) are synthesized and unitized as a powerful theranostic nanoagent for efficient photothermal conversion and multimodal imaging for the first time. We demonstrate that the obtained CoS-PEG NSs show excellent compatibility and stability in water and various physiological solutions, and can be effectively internalized by cells, but exhibit a low cytotoxicity. The CoS-PEG NSs exhibit an efficient photothermal conversion capacity, benefited from the strong near-infrared (NIR) absorption, high photothermal conversion efficiency (~33.0%), and excellent photothermal stability. Importantly, the highly effective photothermal killing effect on cancer cells after exposure to CoS-PEG NSs plus laser irradiation has been confirmed by both the standard Cell Counting Kit-8 and live-dead cell staining assays, revealing a concentration-dependent photothermal therapeutic effect. Moreover, utilizing the strong NIR absorbance together with the T2-MR contrast ability of the CoS-PEG NSs, a high-contrast triple-modal imaging, i.e., photoacoustic (PA), infrared thermal (IRT), and magnetic resonance (MR) imaging, can be achieved, suggesting a great potential for multimodal imaging to provide comprehensive cancer diagnosis. Our work introduces the first bioapplication of the CoS-PEG nanomaterial as a potential theranostic nanoplatform and may promote further rational design of CoS-based nanostructures for precise/efficient cancer diagnosis and therapy.