Concrete-FRP interface plays a critical role in transferring the applied load from the core concrete to fiber in rehabilitated concrete structures. Impregnation of an interfacial defect in the form of void or any anomaly during rehabilitation and/or service life will restrict the retrofitted structure to gain its target strength and eventually will lead to premature failure. The qualitative and quantitative identification of the interfacial void was accomplished satisfactorily in this work. The void identification problem was framed as an optimization-based inverse problem with the purpose of reducing the disparity in the constitutive relation caused by two incompatible stress and strain fields. The disparity originates from the application of two dissimilar constraints in the generation of the stress and strain field. The structural response which facilitates the generation of a kinematically admissible strain field was the surface measurement of the external reinforcement (FRP). The quantitative measure of the interfacial void was parameterized as the damage index in terms of stiffness degradation of the concrete-FRP bond. Numerical validation followed by interfacial characterization with experimental measurement was conducted successfully in this investigation. Detection of the interfacial void in the concrete-FRP interface along with the extent of stiffness degradation quantified as damage index was accomplished in this paper.