Photothermal therapy is a new type of tumor therapy with great potential. An ideal photothermal therapy agent should have high photothermal conversion effect, low biological toxicity, and degradability. The development of novel photothermal therapy agents with these properties is of great demand. In this study, we synthesized boron quantum dots (BQDs) with an ultrasmall hydrodynamic diameter. Both in vitro and in vivo studies show that the as-synthesized BQDs have good biological safety, high photoacoustic imaging performance, and photothermal conversion ability, which can be used for photoacoustic imaging-guided photothermal agents for tumor treatment. Our investigations confirm that the BQDs hold great promise in tumor theranostic applications.