<p>Reinforced concrete (RC) columns, as the principal vertical load-bearing elements of structures, are susceptible to insufficient load-bearing capacity in seismic events and service conditions. This study investigates the axial compressive behaviour of RC columns strengthened with steel tube and ultra-high performance concrete with coarse aggregates (CA-UHPC) composite jackets. One control RC column and six columns strengthened with steel tube and CA-UHPC composite jackets were built and subjected to concentric axial loading. The variables considered in the tests included the thickness of steel tubes, the steel fibre volumetric admixture of CA-UHPC, and the coarse aggregate content of CA-UHPC. The influence of these above variables on the axial behaviour of the columns, including failure mode, load–displacement response, ductility factor, initial stiffness, and energy-absorbing capacity, is analysed and discussed. The results reveal that this strengthening technique led to a more favourable failure mode of the RC columns, effectively preserving their integrity at the ultimate state. Moreover, the axial load-carrying capacity, initial stiffness, and energy absorption capacity of the strengthened RC columns were significantly improved, with maximum increases by factors of 2.58, 3.33, and 5.72, respectively. Coarse aggregate content had a pronounced effect on the elastic modulus of the CA-UHPC and was thus identified as the dominant factor in improving the initial stiffness of strengthened columns. Finally, taking into account the strain lag of the steel tube and CA-UHPC composite jackets, a prediction model for estimating the axial load-carrying capacity of strengthened RC columns is proposed.</p>