This study investigates the synthesis, stability, thermophysical characterization, and thermal performance of water-based Cu-graphene hybrid nanofluids. The Cu-graphene hybrid nanofluid was synthesized by dispersing Cu nanoparticles (0.04 vol%) and graphene nanoplatelets (0.01 to 0.1 vol%) in water. The hybrid nanofluids exhibited excellent stability against aggregation for up to 7 weeks, as evidenced by high zeta potential values. Thermophysical properties such as thermal conductivity, viscosity, density, and specific heat were measured and analyzed. The thermal conductivity of the Cu-graphene hybrid nanofluid showed a significant enhancement of about 35% at low concentrations of hybrid nanostructures. The viscosity of the hybrid nanofluid increased by about 65% compared to water, indicating better fluid transport properties. The specific heat and pumping power decreased with increasing concentration of hybrid nanostructures. A figure of merit (FOM) analysis was conducted to evaluate the thermal efficiency of the hybrid nanofluid under laminar and turbulent flow conditions, showing that the Cu-graphene hybrid nanofluid is most suitable for laminar flow conditions. The study highlights the potential of Cu-graphene hybrid nanofluids as an efficient heat transfer fluid in various applications.This study investigates the synthesis, stability, thermophysical characterization, and thermal performance of water-based Cu-graphene hybrid nanofluids. The Cu-graphene hybrid nanofluid was synthesized by dispersing Cu nanoparticles (0.04 vol%) and graphene nanoplatelets (0.01 to 0.1 vol%) in water. The hybrid nanofluids exhibited excellent stability against aggregation for up to 7 weeks, as evidenced by high zeta potential values. Thermophysical properties such as thermal conductivity, viscosity, density, and specific heat were measured and analyzed. The thermal conductivity of the Cu-graphene hybrid nanofluid showed a significant enhancement of about 35% at low concentrations of hybrid nanostructures. The viscosity of the hybrid nanofluid increased by about 65% compared to water, indicating better fluid transport properties. The specific heat and pumping power decreased with increasing concentration of hybrid nanostructures. A figure of merit (FOM) analysis was conducted to evaluate the thermal efficiency of the hybrid nanofluid under laminar and turbulent flow conditions, showing that the Cu-graphene hybrid nanofluid is most suitable for laminar flow conditions. The study highlights the potential of Cu-graphene hybrid nanofluids as an efficient heat transfer fluid in various applications.