Here, we report a one-pot, straightforward, and efficient method for the covalent functionalization of black phosphorous (BP) nanosheets with biopolymers on a gram scale. L-lactide and glycidol monomers were stepwise copolymerized onto the surface of exfoliated BP resulting in highly water-soluble functional nanosheets (BP-PLA-PG). In contrast to pristine BP, their functionalized counterparts did not degrade under ambient conditions and were stable for several weeks. Owing to their stability, efficient photothermal effect, and low toxicity, BP-PLA-PG sheets were loaded with doxorubicin (DOX), and their anticancer effect was investigated in vitro and in vivo. The laser-triggered release of DOX, in combination with the photothermal effect of the functionalized BP sheets, manifested in strong antitumor effects in animal studies. One-pot straightforward functionalization, high loading capacity, low toxicity, excellent water dispersibility and stability, as well as high functionality, support BP-PLA-PG as a promising vector for the future biomedical applications, including targeted drug delivery and bioimaging.