Several destructive earthquakes in developing countries in recent years have revealed that a large number of reinforced concrete (R/C) buildings contained no lateral reinforcements in beam-column joints. Severe damage was caused to such beam-column joints due to poor structural capacities, and resulted in complete/story collapse of buildings and loss of human lives. Considering the economic and technical conditions in developing countries, this paper proposes a practical seismic strengthening method for applying R/C wing walls to this kind of substandard beam-column joint. A feasible design concept is presented exemplifying an exterior beam-column joint, representing a typical earthquake-damaged joint by the 2009 West Sumatra, Indonesia earthquake. In this study, two 3/4-scale exterior beam-column joint specimens were constructed with the common structural details, and one of them was strengthened by R/C wing walls. Their seismic performance was evaluated through static cyclic loading tests. It was found that the strengthened specimen behaved a ductile manner with beam yielding, whereas the unstrengthened control specimen prematurely failed at the joint. The proposed strengthening method significantly increased the moment resistance of the joint. Fundamental experimental data could be successfully obtained to propose the calculation procedure for designing R/C wing walls for practical strengthening.