The experimental validation of the concept of selective weakening (SW) for seismic retrofit of existing pre-1970s reinforced concrete frames is herein presented. The SW retrofit strategy is to modify the brittle inelastic mechanism to a more ductile mechanism by first weakening selected parts of the structure. Subsequently, the structure can be further upgraded to the desired strength/stiffness/ductility and energy dissipation capacity. Different levels of performance are achievable, from collapse prevention to damage control. For a beam-column (bc) joint, the proposed SW retrofit involves severing the bottom longitudinal reinforcement of the beam, and if required, adding external post-tensioning tendons. In this paper, the experimental implementation of the SW retrofit for poorly detailed exterior bc joint subassemblies is presented. Four 2/3 scaled exterior bc joint subassemblies are used to investigate the feasibility and effectiveness of selective weakening retrofit. Generally, the experimental results confirm previous numerical findings of the viability of SW retrofit to improve seismic performance of existing bc joints. By reducing the shear demand through beam weakening and/or increasing the joint capacity by adding horizontal axial load from external post-tensioning, the local inelastic mechanism is concentrated to a ductile flexural beam hinge, thus achieving the desirable weak-beam strong column/joint global mechanism. Complementing this paper are earlier numerical results of refined FEM 3D models of the exterior bc joint and macro-model of a multi-storey prototype structure.