The rapid development and broad utilization of nuclear technologies has raised safety concerns, especially for those requiring radiation‐shielding equipment. To cope with such high demands and risks, the current study investigated the potential of utilizing ultra‐high‐molecular‐weight polyethylene (UHMWPE) composites containing varying contents of surface‐treated samarium oxide (Sm2O3) or gadolinium oxide (Gd2O3) particles as dual thermal neutron‐ and gamma‐shielding materials. The results showed that the thermal neutron‐ and gamma‐shielding abilities of the composites increased with increasing filler contents, as evidenced by the highest values of μ (in the case gamma rays), ∑t (in the case of thermal neutrons), and μm, as well as the lowest values of HVL and TVL, being achieved in the samples containing 25 wt% Sm2O3 or Gd2O3 (the maximum content investigated). Furthermore, based on the comparative thermal neutron‐shielding properties of the current composites to those of a common shielding product containing 15.9 wt% B2O3 (corresponding to 5 wt% B), the addition of 2.5 wt% Gd2O3 or 4.8 wt% Sm2O3 to the UHMWPE matrix was sufficient to attenuate thermal neutrons with equal efficiency to that of the referenced material. Furthermore, the results indicated that the addition of both fillers increased the density and hardness (Shore D) but reduced the degree of crystallinity, tensile strength, elongation at break, and dielectric strength of the composites, for which the degree of changes in each property largely depended on filler types and contents. In summary, the overall results suggested that both Sm2O3 and Gd2O3 could substantially enhance the thermal neutron‐ and gamma‐shielding properties of the composites, while the developed UHMWPE composites offered durability and excellent electrical properties that could be suitable for use in various applications.