Abstract: Black phosphorus nanosheets (BPNSs) hold great promise for biomedical applications but suffer from poor environmental stability. Herein, we stabilize BPNSs via electrostatic passivation using adenine-modified quaternized chitosan (HACC-A), where the permanent cationic groups interact with the lone-pair electrons of BPNSs to form BP-HACC-A complexes, reducing the 7-day degradation rate from 66.32% to 37.36%. These complexes are then used as precursors to construct a dynamic covalent hydrogel (BP@Gel) through Schiff-base reaction between the amino groups of HACC-A and an eight-armed POSS-PEG-CHO crosslinker. In this system, HACC-A plays a dual role as both a BPNSs stabilizer and a hydrogel backbone, BPNSs act as a functional additive providing near-infrared (NIR) photothermal conversion, and POSS-PEG-CHO contributes to mechanical reinforcement and self-healing. Encapsulation within the hydrogel further lowers the degradation rate of BPNSs to 18.45% (7 d) and 29.63% (14 d). The resulting hydrogels exhibit a uniform porous structure, tunable swelling (900–1300%), rapid gelation (~200 s), high storage modulus (2300 Pa), self-healing, injectability, and strong tissue adhesion (36.2 kPa). Under 808 nm laser irradiation, BP@Gel shows concentration- and power-dependent photothermal heating (up to 47 ℃) with excellent cycling stability. Antibacterial assays reveal that without NIR irradiation the antibacterial rates are 42.6% (E. coli) and 31.8% (S. aureus), which increase dramatically to 96.8% and 92.4% upon NIR exposure, demonstrating a photothermal-enhanced “heat-boosted chemical killing” effect. The hydrogel also displays good cytocompatibility. By clearly defining the role of each component and their synergy, this work provides a new strategy for stabilizing BPNSs and constructing multifunctional hydrogels.