The influence of functionalised graphene nanoplatelets with melamine on the thermal and mechanical properties of a 3D printed photopolymerisable resin is investigated. In this work, a liquid-based 3D printer, stereolithography, was employed to fabricate the 3D printed parts, and a commercial dimethacrylate-based resin was used. The 3D printed parts were subjected to ultraviolet and thermal post-curing stages to improve thermal and mechanical behaviour. The quality of the graphene nanoplatelets functionalisation was characterised by Fourier transform infrared spectroscopy and thermogravimetric analysis. Thermal and mechanical characterisation of the 3D printed nanocomposites were performed via thermogravimetric, tensile and Izod impact tests. The fractured surfaces were observed via scanning electron microscopy. The degree of graphene nanoplatelets dispersion in the polymer matrix is enhanced by bonding with melamine via π-π interactions and inhibited surface defect formation. Results show property enhancements of up to 35% in tensile strength, 78% in impact strength and 38% in residual weight at 400 °C. The improvement in thermal stability, tensile and impact properties with such a small amount of functionalised graphene nanoplatelets is owing to the strong interfacial adhesion between functionalised graphene nanoplatelets and the 3D printed polymer matrix. This in turn enhanced the UV curing reaction with the dimethacrylate-based photocurable resin.