In this study, a novel ultra-high-performance concrete (UHPC) composite plate that is reinforced with a fibre-reinforced polymer (FRP) grid (herein FRP-UHPC composite plate) is developed and reported. The durability and mechanical performance of FRP-UHPC composite plates in harsh environments are expected to be superior to conventional materials when used in optimal configurations; because i) both UHPC and FRP composites in isolation have excellent mechanical properties in specific directions, and ii) they are also durable materials. The flexural and tensile behaviour of FRP-UHPC composite plates, with and without the inclusion of steel fibres in the UHPC mix, were investigated via experimentation. The test results, which demonstrate the excellent interaction between FRP and UHPC, confirm the viability of the system: i) the inclusion of an FRP grid enhances the ultimate flexural capacity by over 150% and the ultimate tensile capacity by over 200%; ii) the FRP-UHPC composite plates exhibit tensile elastic-strain hardening behaviour and the average ultimate tensile stress of the composite plates with an FRP reinforcement ratio of about 0.69% is over 25 MPa; and iii) the interaction between the FRP grid and UHPC is sufficient to transfer stresses in the longitudinal FRP strips to the UHPC through the transverse strips of the FRP grid, while the steel fibres in the UHPC passing through the openings of the FRP grid reduce the likelihood of FRP debonding. The proposed strain-hardening FRP-UHPC plates are expected to be promising for structural elements with various purposes.