Black phosphorus quantum dots (BPQDs) were synthesized using a liquid exfoliation method that combined probe sonication and bath sonication. With a lateral size of approximately 2.6 nm and a thickness of about 1.5 nm, the ultrasmall BPQDs exhibited an excellent NIR photothermal performance with a large extinction coefficient of 14.8 Lg−1 cm−1 at 808 nm, a photothermal conversion efficiency of 28.4 %, as well as good photostability. After PEG conjugation, the BPQDs showed enhanced stability in physiological medium, and there was no observable toxicity to different types of cells. NIR photoexcitation of the BPQDs in the presence of C6 and MCF7 cancer cells led to significant cell death, suggesting that the nanoparticles have large potential as photothermal agents. A liquid exfoliation method based on a combination of probe sonication and bath sonication was adopted to synthesize black phosphorus quantum dots (BPQDs). These displayed a high extinction coefficient of 14.8 Lg−1 cm−1, a photothermal conversion efficiency of 28.4 %, and good biocompatibility, and can thus be used as highly effective photothermal agents for cancer therapy.