This paper presents a new strengthening technique for square-reinforced concrete (RC) columns by circularisation with reactive powder concrete (RPC) and wrapping with fibre-reinforced polymer (FRP). RC column specimens were tested, divided into four groups of four specimens based on the strengthening technique: four reference square specimens (150mm side length) without any strengthening, four were wrapped with two layers of carbon fibre-reinforced polymer (CFRP) and the remaining eight were strengthened by changing the square cross-section to a 240mm diameter circle with RPC jacket. Four of the RPC jacketed specimens were left unwrapped, while the last four were wrapped with two layers of CFRP. From each group, one specimen was tested under concentric axial load, two were tested under eccentric axial load and one was tested under four-point bending. It was found that using the RPC for circularisation and strengthening of existing square RC columns is an effective technique to significantly increase their axial carrying capacity, ultimate flexural load and energy absorption. Wrapping the circularised RC columns with CFRP prevented the failure of the RPC jacket at the corners of the existing square RC columns under the axial load, and improved the ultimate load as well as the energy absorption of the circularised RC columns.