Abstract: To investigate the torsional behavior of reinforced concrete (RC) beams strengthened with ultra-high-performance concrete (UHPC), this study conducted pure torsion tests on 9 specimens, including one un-strengthened RC beam and eight UHPC-strengthened RC beams. Additionally, refined finite element models were established using ATENA software to simulate the full-range mechanical behavior of the specimens under pure torsion. Through a combination of experimental and numerical results, the enhancement mechanism of UHPC on the torsional performance of RC beams were explored, and the influence of each variable parameter on the torsional performance of the strengthened beams were examined. The results indicate that the use of UHPC significantly enhances the ability of RC beams to resist torsional moments. The failure mode of specimens strengthened on two sides can lead to serious safety risks, and it is therefore not recommended in actual strengthening projects. If there are strict size restrictions on the specimens after strengthening, the three-sided wrapping scheme can serve as an alternative to the four-sided wrapping scheme. Prior to strengthening, it is necessary to roughen the surface of RC beams to ensure a good bonding performance between the UHPC layer and the RC beam. The decision to incorporate steel bars into the UHPC layers must not only consider the enhancement in torsional capacity but also account for the constraints in the post-strengthening dimensions of the RC beam. This study comprehensively compared the strengthening method using UHPC with that using FRP in terms of the increase in torsional capacity, of the enhancement in torsional stiffness, of the failure modes, of the economic efficiency and, of the construction convenience. The research findings in this study could be used as a reference for strengthening projects in the future.