The convective heat transfer coefficient and friction factor for fully developed turbulent flow of graphene nanoplatelet (GNP) nanofluids of varying specific surface areas flowing through a horizontal stainless steel tube with a uniform heat flux are experimentally determined. The thermo-physical properties of the GNP nanofluid, including the thermal conductivity and viscosity, are measured at various temperatures, specific surface areas (300, 500, and 750 m2/g) and concentrations (0.025, 0.05, 0.075, and 0.1 wt%). To validate the reliability and reproducibility of the experimental setup for calculating the convective heat transfer coefficient and for providing a baseline to compare the GNP nanofluid data, several tests are conducted for distilled water. The main objective is to evaluate the effect of specific surface area of GNP nanoparticles for varying concentrations on heat transfer under turbulent flow conditions. The convective heat transfer coefficient of the GNP nanofluid is found to be higher than the base fluid by approximately 83–200%. Further, the heat transfer coefficient of the GNP nanofluid increases as the flow rate and the specific surface area increase. However, the pressure drop increases simultaneously in the range 0.06% to 14.7%. The results suggest that GNP nanofluids could function well as working fluids in heat transfer applications and provide good alternatives to conventional working fluids.