Flow and heat transfer characteristics of gas coolers, one of the most significant components in transcritical CO2 heat pumps, significantly influence the overall performance of the heat pumps. To ensure high heat transfer efficiency and increased structural stability of gas coolers, a novel biomimetic honeycomb fractal heat exchanger is proposed in this study. The conjugate heat transfer between supercritical CO2 and water in this novel heat exchanger is then investigated by numerical methods under different operating and geometric parameters. The results indicated that the heat transfer coefficients of CO2 were quite different in different flow channels and different locations in each flow channel, especially near the supercritical region, which was significantly influenced by the buoyancy effect. Based on the thermophysical parameters, buoyancy, and hydraulic diameter, a new correlation for supercritical CO2 was developed. The relative errors between the data calculated by the corre