This paper proposes a novel retrofitting system for the strengthening of steel I-beams using prestressed unbonded carbon fiber reinforced polymer (CFRP) plates. The system relies on a pair of mechanical clamps. Each clamp holds two flat CFRP plates (each 50 × 1.4 mm) and anchors their prestressing forces to the steel I-beam via friction. The proposed system does not require any adhesive between the CFRP plates and the steel substrate; therefore, surface preparation and adhesive curing are not needed, which reduces the time and cost of retrofitting. The design considerations of the mechanical clamps, the most important elements of the proposed unbonded system, are explained. A finite element (FE) model was established to optimize the design of the required mechanical parts, and a set of pull-off tests was performed to evaluate the capacity of the optimized system. The experimental results show that the proposed system is capable of transferring almost the entire tensile capacity of the normal modulus CFRP plates to the steel substrate via friction, while no sliding of the joint was observed. Moreover, the proposed flat prestressed unbonded reinforcement (FPUR) system was applied on a 6.4-m-long steel I-beam and the high performance of the system was confirmed based on the results of a set of four-point static and fatigue tests. Relying on the laboratory experimental tests, the proposed FPUR system was applied on the cross-girders of an almost 120-year-old steel bridge in Australia to monitor its long-term performance as a strengthening solution to increase the capacity and/or fatigue life of existing metallic I-beams.