Supramolecular polymeric gels cross-linked by well-defined, discrete metal-organic macrocycles (MOMs) or metal-organic cages have become a prevailing topic within the field of supramolecular self-assembly. However, the realization of supramolecular polymeric hydrogels cross-linked by discrete organometallic architectures with good biocompatibility is still a great challenge. Herein, we present the successful preparation of CO2 stimuli-responsive, injectable block copolymer hydrogels cross-linked by discrete organoplatinum(II) metallacycles. Through the combination of coordination-driven self-assembly and stepwise post-assembly polymerization, star block copolymers (SBCPs) containing well-defined hexagonal metallacycles as cores were successfully prepared, which featured CO2 stimuli-responsive properties including CO2-triggered morphology transition and CO2-induced thermoresponsive behavior. Interestingly, the resultant SBCPs were capable of forming supramolecular hydrogels with MOMs as junctions near physiological temperature, which allowed the realization of a reversible gel-to-sol transformation through the removal and addition of CO2. More importantly, the resultant supramolecular hydrogels presented good cytocompatibility in vitro. Therefore, this study provides a new strategy for the construction of new "smart" supramolecular hydrogels with promising applications as biological materials.