Strengthening of existing steel members using prestressed carbon fiber reinforced polymer (CFRP) plates has certain advantages over non-prestressed reinforcement, especially due to the reduction in permanent tensile stresses acting on the steel members. However, only very low prestressing levels in CFRP plates may be reached when adhesively-bonded reinforcement is used. The reason is that high interfacial shear stresses generated in the adhesive layer during the prestress release process lead to the premature debonding of the prestressed CFRP reinforcements from the steel substrate under transient service loads. In the current study, a novel unbonded mechanical clamp is proposed to anchor high prestressing forces to the steel substrate. A finite element (FE) model was used to optimize the design of the required mechanical parts. Subsequently, a set of static and fatigue tests was performed to evaluate the performance of the optimized system. The experimental results proved that the proposed system is capable of transferring the entire tensile capacity of the normal modulus CFRP reinforcing plates (50 × 1.4 mm each) to the steel substrate without any slippage of the joint. Consequently, based on the static and fatigue tests performed in the current study, it can be concluded that the proposed unbonded mechanical clamping system can be used for strengthening of existing steel structures using prestressed unbonded reinforcements (PURs) to increase the ultimate capacity and/or fatigue life.