An estimated volume of 14 million m3 of the removed soil and waste generated from decontamination projects in Fukushima Prefecture is being transported to the interim storage facility at present. Modification agents such as water-absorbing polymer are used to improve the treatability of the removed soil for efficient separation of foreign materials. It is expected that the modified removed soil can be reused as geomaterial to reduce the final disposal volume under the major premise of safety assurance. Thus, it is important to understand the mechanical properties of the modified removed soil and the influence of the modification agents. This study focused on the compressibility and the unconfined compressive strength of the modified removed soil. Compression tests using oedometer and unconfined compression tests were conducted on the removed soil modified with a water-absorbing polymer powder agent and the mix proportion by weight was 3% and 10%. Decomposed granite soil was used as a comparison. With addition of 10% polymer agent, the final settlement of the modified removed soil decreased by 20% and the unconfined compressive strength increased by 80% to 82 kN/m2. The results suggest that the mechanical performance of the modified removed soil might be improved with a certain amount of polymer agent addition.