•Performance-based design optimization of steel moment frames is implemented.•Local and overall damage indices are calculated for the optimal frames.•Overall damage index-based fragility curves are generated.•A new damage indicator termed as damage margin ratio is proposed.•Repairability borders are determined for low-, mid-, and high-rise optimal steel moment frames. This study is devoted to seismic performance assessment of optimally designed steel moment frames (SMFs) in the framework of performance-based design (PBD). The methodology presented in this work includes three phases. The first phase involves the optimization of SMFs by employing an efficient metaheuristic algorithm to meet the PBD requirements according to FEMA-350 code. Subsequently, the overall damage index (ODI) is calculated for the obtained optimal SMFs based on the Park-Ang local damage index (DIPA). In the second phase, incremental dynamic analysis (IDA) is conducted for the optimally designed SMFs and their fragility curves are derived and their collapse margin ratios (CMRs) are determined based on FEMA-P695. In the last phase, the fragility curves of the optimal SMFs are generated for different damage levels ranging from slight damage to collapse state and a new damage measure termed as damage margin ratio (DMR) is introduced to assess the damage-resistance capacity of the SMFs at the different damage levels. In order to illustrate the efficiency of the proposed methodology, three numerical examples of 3-, 6-, and 12-story SMFs are presented and the total cost of optimal SMFs, including initial and seismic damage costs, are determined. The numerical results demonstrate that the SMF with the best total cost has the best CMR, DMR, and degree of repairability.