Synthesis of phase pure hopeite pigment through a solvothermal flow method is reported here for the first time. The products show two-step dehydration behaviour from thermogravimetric analysis (TGA), and a higher degree of purity and homogeniety than commercial zinc phosphate pigment. By increasing the reaction temperature stepwise from room temperature to 350 °C it was possible to decrease the size of the individual crystallite sheets and to tune their packing into larger assemblies. The conversion of reactants to product proved to be significantly higher at increased temperature with a measured yield of 98.7% at 250 °C versus 85.4% at room temperature. The synthesis route demonstrated here is environmentally sustainable, increases cost-efficiency through minimization of waste, and is compatible with a scale-up strategy.