Fibre-reinforced polymers (FRPs) are widely used for strengthening and rehabilitating ageing concrete structures, with their effectiveness strongly influenced by the behaviour at the FRP–concrete interface. This study presents a closed-form analytical solution for predicting the shear response of FRP-retrofitted concrete systems, derived from a bond–slip model governed by interfacial Mode II fracture energy. Unlike many existing models, the proposed approach explicitly incorporates principles of fracture mechanics, thereby improving both its physical relevance and predictive accuracy. The model is validated using bond–slip parameters obtained from the literature and calibrated against experimental results from previous studies. The comparisons show good agreement, confirming the model’s ability to capture the load-slip response of retrofitted systems. By anchoring the analysis in fracture energy, the formulation offers a generalisable and robust tool for assessing interfacial debonding.