Photothermal therapy (PTT) is an emerging technology as a noninvasive therapeutic modality for inducing photonic cancer hyperthermia. However, current photothermal conversion agents suffer from low therapeutic efficiency and single functionality. Engineering crystal defects on the surface or substrate of semiconductors can substantially enhance their optical absorption capability as well as improve their photothermal effects in theranostic nanomedicines. In this study, a specific defect engineering strategy was developed to endow two-dimensional (2D) BiOCl nanosheets with intriguing photothermal conversion performance by creating oxygen vacancies on the surface (O-BiOCl). Importantly, the photothermal performance and photoacoustic imaging capability of the 2D O-BiOCl nanosheets could be precisely controlled by modulating the amounts of oxygen vacancies. The strong Bi-based X-ray attenuation coefficient endowed these nanosheets with the contrast-enhanced computed tomography imaging capability. The high near-infrared-triggered photonic hyperthermia for tumor ablation was systematically demonstrated both in vitro at the cellular level and in vivo for tumor breast cancer mice xenograft models. Based on the demonstrated high biocompatibility of these 2D O-BiOCl nanosheets, this work not only formulates an intriguing 2D photothermal nanoagent for tumor ablation, but also provides an efficient strategy to control the photothermal performance of nanoagents by defect engineering.