Single component agent-based near-infrared phototheranostics has captivated researchers in cancer treatment over recent decades. However, the current single-component agents rarely alleviate the hypoxic microenvironment in tumor cells, which severely limits the development of photodynamic therapy. Here, we report a new phototheranostic agent, single-component black graphite carbon nitride nanoparticles (named CN-B NPs), synthesized through thermal copolymerization. Compared with unmodified graphitic carbon nitride (g-C3N4), the absorption of CN-B NPs in the near-infrared region was enhanced. Through in vitro and in vivo studies, we demonstrated that CN-B NPs can be used as a near-infrared phototheranostic agent in both normoxia and hypoxia, simultaneously allowing infrared thermal/photoacoustic imaging and photodynamic/photothermal co-therapy, thereby resulting in remarkable inhibition of tumor growth. This approach could provide a new method to achieve multimodal phototheranostics using a single-component material.