Abstract Poly(methyl methacrylate) (PMMA) was reinforced by using colemanite Ca2B6O11·5H2O (CMT) to increase the radiation shielding capacity of the polymer composite. CMT reinforcement effect was examined at four different CMT concentration levels (such as 5, 15, 30 and 40 wt. %) to investigate radiation protective shielding properties of PMMA/CMT composite. The effect of the CMT reinforcement on the radiation shielding capacity was determined to use the composite at the mixed fields of neutrons and gamma rays. For this purpose, the polymer composite samples were prepared by using in-situ polymerization with atom transfer radical polymerization (ATRP) technique. Radiation shielding capacity was experimentally assessed by using gamma and neutron attenuation tests. The gamma-ray shielding capacity of the composite was examined by using Cs-137 radioisotope and the experimental results of gamma transmittance were compared with theoretical calculations by using WinXCOM. Neutron transmission tests were performed by using a neutron howitzer (239Pu–Be neutron source). Radiation shielding performance of PMMA/CMT composite was examined by the determination of the attenuation coefficient (μ) and the half-value layer (HVL). CMT reinforcement of PMMA supported to increase the radiation shielding capacity by 11.1% for gamma photons of Cs-137 radioisotope and 38.56% for neutrons emitted from Pu–Be neutron source.