Indocyanine green (ICG) is the only near-infrared (NIR) dye approved by the United States Food and Drug Administration (FDA). Although it is highly desirable, some bottlenecks still remain in clinical applications, such as poor photostability, poor thermal stability, and lack of target specificity. To solve these problems, a zeolitic imidazolate framework-8 (ZIF-8)-based ICG theranostic agent was constructed by one-pot synthesis for fluorescence imaging and photothermal therapy (PTT). The as-synthesized ICG@ZIF-8 nanoparticles (NPs) displayed ultrahigh loading capacity, superior photothermal stability, good anti-photobleaching ability, good biocompatibility, efficient cellular uptake, and favourable photothermal killing capacity for human hepatocarcinoma SMMC-7721 cells. Importantly, in vivo experiments showed that ICG@ZIF-8 NPs accurately and sensitively detected tumors by fluorescence molecular imaging. The PTT results indicated that ICG@ZIF-8 NPs efficiently induced a local ablation effect under a single NIR laser irradiation. The tumor was completely suppressed, and no tumor recurrence or treatment-induced toxicity was observed. The described particles have the potential to act as a promising platform for cancer theranostic nanomedicine.