This study presents an enhancement in the heat transfer performance of a glass thermosyphon using graphene–acetone nanofluid with 0.05%, 0.07%, and 0.09% volume concentrations. The heat load is varied between 10 and 50 W in five steps. The effect of heat load, volume concentration, and vapor temperature on thermal resistance, evaporator and condenser heat transfer coefficients, are experimentally investigated. A substantial reduction in thermal resistance of 70.3% is observed for the maximum concentration of 0.09% by volume of graphene–acetone nanofluid. Further, an enhancement in the evaporator heat transfer coefficient of 61.25% is observed for the same concentration. Also from the visualization study the different flow patterns in the evaporator, adiabatic, and condenser regions are obtained for acetone at different heat inputs.