A novel gapped eccentric steel brace (GESB) was proposed to achieve a two-stage retrofit strategy for seismic upgrading of reinforced concrete (RC) frames. The steel brace was isolated from the RC frame at low story drifts using thin layers of polymer cellular materials, while the brace was activated at high drifts. Mild steel coupling beam was introduced into the brace system as a damper to improve the energy dissipation capacity of the system. Four frames were subjected to pseudo-static reversed cyclic loading procedure, including one bare frame and three retrofitted frames with the novel system. The experimental results indicated that the GESB system presented an obvious two-stage behavior which was generally superior in terms of providing second-stage enhancement of stiffness while avoiding drastic intervention in the initial vibration characteristics of the primary structure. Besides, the yield of mild steel beam greatly increased the energy dissipation capacity of the system. Compared with the reference frame, cracks were more diffuse in the retrofitted frames rather than concentrated at the column end, which means all of the original structure elements were fully used. [ABSTRACT FROM AUTHOR]