In this study, the seismic behaviors of strengthened three-dimensional frames, as in real-life structures, are presented. Three reinforced concrete (RC) frames, containing common structural deficiencies were constructed to represent existing older structures. The bare, damaged, and undamaged frames, strengthened by RC external shear walls with steel tie beams, were tested under a reversed cyclic load. The experimental results indicated that strengthening by external shear walls made significant contributions to the frames in lateral strength, stiffness, and energy dissipation capacity. This proposed method appears to be an efficient technique for strengthening structures in an effective, economical, and practical way.