The development of effective approaches for the preparation of 0D quantum dots (QDs)/2D nanosheets (NSs) heterostructures, which have been proven to be favorable for heterogeneous catalysis, is highly desirable but remains a great challenge. Herein, 0D metal oxide nanocrystals–2D ultrathin g‐C3N4 nanosheets (Co3O4/CNNS) heterostructures are synthesized via a facile chemical reaction, followed by annealing in air. Ultrafine Co3O4 QDs (≈2.2–3.2 nm) are uniformly and tightly attached on the surface of g‐C3N4 nanosheets. Detailed characterization reveals that the specially designed unique 0D/2D structure is critical to the high photocatalytic performance for the degradation of tetracycline (TC) via peroxymonosulfate (PMS) activation. The optimal catalyst, namely, Co3O4/CNNS‐1100, exhibits excellent performance and ≈98.7% TC can be degraded under visible light irradiation. Moreover, TC degradation is almost completely insusceptible to several real water samples. Meanwhile, other dye pollutants can also be efficiently degraded by the Co3O4/CNNS‐1100/PMS/vis system. The quenching tests display that that the h+, ∙OH, O2∙−, and SO4∙− are responsible for TC removal. The improved photocatalytic performance can be attributed to the synergistic effect of the photocatalytic‐ and chemical‐processes in the PMS activation. This work gives an insight into the development of multifunctional 0D/2D nanocomposites for further potential applications which are not limited to environmental purification. Co3O4 quantum dots are strongly coupled with graphitic carbon nitride nanosheets using an in situ growth strategy. Such structures efficiently prevent the aggregation of Co3O4 and permit efficient interfacial electrons transport (ascribed to the large contact area provided by the 0D–2D structure), synchronously the photocatalytic‐ and chemical‐processes, so to render enough active sites to make peroxymonosulfate activation persistent.