An experimental investigation was performed to evaluate the heat transfer characteristics and the pressure drop of a graphene nanoplatelet (GNP) nanofluid in a horizontal stainless steel tube that was subjected to a uniform heat flux at its outer surface. The thermal conductivity and viscosity of the GNP nanofluids at concentrations of 0.025, 0.05, 0.075, and 0.1 wt % were measured prior to the heat transfer experiments. The heat transfer and the pressure drop within the flowing base fluid (distilled water) were measured and compared with the corresponding data from the correlations. The data were satisfied within a 5% error and a 95% confidence level. The effects of the nanoparticle concentration and the heat flux on the enhancement of the heat transfer turbulent flow condition are presented. The convective heat transfer coefficient of the GNP nanofluid is higher than that of the base fluid by approximately 13−160%. Further, the heat transfer coefficient of the GNP nanofluid increased as the flow rate and the heat flux increased. However, the increase in the pressure drop ranged from 0.4% to 14.6%. Finally, an analysis of the thermal performance factor reveals that the GNP nanofluids at concentrations of 0.075 and 0.1 wt % could function as a good and alternative conventional working fluid in heat transfer applications.