This paper presents the determination of an appropriate earthquake damage index for infill masonry walls in reinforced concrete frame structures. The index can be used to determine the level of damage to the infill wall under earthquake loading. The damage index can be obtained from structural analysis of the infilled RC frame with an appropriate model for the frame and infill elements using the Park–Ang formula. The proposed damage index was calibrated with the test results collected from previous studies. The data used in this study were based on cyclic tests of RC frames with masonry walls subjected to monotonic and horizontal cyclic loads. The behavior of the infilled wall was stimulated using two equivalent compression-only struts. The analysis results, including the displacement and energy dissipation, were used to assess the damage that occurred to the infilled wall. The proposed damage index can be used in the performance assessment of reinforced concrete frames with masonry walls.