This paper proposes a seismic retrofitting technique for existing RC columns with cast-in-place RC sidewall using polymer-cement mortar (PCM). In the technique, additional shear reinforcement (deformed steel bar: prefabricated reinforcing unit) is adhered to the surface of existing RC columns with RC sidewall by PCM. Shear-loading tests were conducted to clarify the structural performance of the columns with sidewall strengthened by polymer-cement mortar. The results suggested that the capacity and ductility of the strengthened RC column with RC sidewall could be controlled by choosing the strengthened area and the amount of shear reinforcement. Furthermore, the shear force Q &mdash drift angle R curve of RC columns with RC sidewalls retrofitted by RCM was proposed by considering the shear resistance mechanism.