Reinforced concrete (RC) columns are vital components in the construction of large-scale coastal structures, including bridges and piers. Over time, exposure to marine and nearshore environments can lead to a significant deterioration of RC columns. This study introduces an innovative method for strengthening circular concrete columns using two semi-circular shells made from fiber-reinforced polymer (FRP) bar reinforced ultra-high-performance concrete (UHPC), referred to as FRU shells. These semi-circular FRU shells can be easily assembled on-site by connecting them with grouting sleeves, forming a circular tubular jacket that encloses the existing concrete columns. In this study, compression tests were conducted to assess the effectiveness of the proposed strengthening technique. The influence of external FRU jackets with different FRP spacing and various jacket heights was discussed. The results demonstrate that this strengthening approach significantly improves the initial stiffness, ultimate load-bearing capacity, and ductility of the existing circular concrete columns. A theoretical model for predicting the compressive strength of the strengthened columns is proposed and verified against the test results. The proposed prefabricated strengthening system has the potential to be implemented in an underwater environment and to streamline in-situ construction processes for strengthening of various kinds of columns.