All-in-one nanoagents with a single-component and all-required functions have attracted increasing attention for the imaging-guided therapy of tumors, but the design and preparation of such nanoagents remain a challenge. Herein, we report the introduction of oxygen vacancies to traditional semiconductors with heavy-metal elements for tuning photoabsorption in the near infrared (NIR) region, by using Bi₂WO₆ (band-gap: ∼2.7 eV) as a model. Bi₂WO_6−x nanodots with sizes of ∼3 or ∼8 nm have been prepared by a facile coprecipitation-solvothermal method assisted by citric acid (CA, 0.1–1.5 g) as the reduction agent. CA confers the removal of O atoms from the [Bi₂O₂]²⁺ layer during the solvothermal process, resulting in the formation of plenty of oxygen vacancies in the Bi₂WO_6−x crystal. As a result, NIR photoabsorption of Bi₂WO_6−x nanodots can be remarkably enhanced with the increase of the CA amount from 0 to 1.0 g. Under irradiation of a single-wavelength (808 nm, 1.0 W cm⁻²) NIR laser, black Bi₂WO_6−x-CA_1.0 nanodots can not only efficiently produce a sufficient amount of heat with a photothermal conversion efficiency of 45.1% for photothermal therapy, but also generate singlet oxygen (¹O₂) for photodynamic therapy. Furthermore, due to the presence of heavy-metal (Bi and W) elements, Bi₂WO_6−x-CA_1.0 nanodots have high X-ray attenuation ability for CT imaging. After the Bi₂WO_6−x-CA_1.0 nanodot dispersion is injected into the tumor-bearing mice, the tumor can be imaged by using CT and an IR thermal camera. After irradiation with a single-wavelength (808 nm, 1.0 W cm⁻², 10 min) NIR laser, the tumor can be completely suppressed by the synergic photothermal and photodynamic effects of Bi₂WO_6−x-CA_1.0 nanodots, without recurrence and treatment-induced toxicity. Therefore, Bi₂WO_6−x nanodots have great potential as a novel all-in-one nanoagent for the imaging and phototherapy of tumors.