This study emphasizes a new strengthening scheme for seismically vulnerable flat plate-column connections in existing structures. An experimental investigation on a series of three half-scaled interior plate-column connections was conducted to explore the effectiveness of the proposed seismic strengthening method using postinstalled RC wing walls and to analyze the punching shear capacity of the connections. This study focuses on typical existing flat plate structures constructed using low-strength concrete in Bangladesh. Two of the three specimens had wing walls employed in two principal directions, that is, along the loading direction and orthogonal direction, while another specimen was tested without wing walls and served as a control specimen for comparisons between the cases with and without strengthening. Static cyclic vertical displacements with increasing amplitude were applied to the ends of the slabs to represent interstory drift under seismic action. Both strengthened specimens showed higher resistance to punching shear failure. Thus, the test results experimentally verified that installing RC wing walls is a feasible method to upgrade flat plate-column connections, including those constructed using low-strength concrete, to prevent collapse under seismic action.