<p>This paper presents the results of an experimental and numerical study of the mechanical behavior of thermoplastic polyurethane (TPU) manufactured using 3D printing (FDM), with an emphasis on its elastic, viscoelastic, and relaxation properties. For this purpose, a series of uniaxial tensile tests was conducted at different loading rates, enabling the dependence of the material’s mechanical response on deformation rate to be established. It was found that both the elastic modulus and the ultimate strength increase with increasing speed, indicating the need to account for speed effects in dynamic calculations. In addition, relaxation tests were performed at different deformation levels, indicating that the load level significantly influences the relaxation process, while the initial deformation rate has a negligible effect. A Proné series model was used to approximate the obtained relaxation curves. It was determined that a two-component approximation is optimal, as increasing the number of terms leads to unrealistic time parameters. To verify the correctness of the obtained characteristics, numerical modeling was performed under conditions that reproduced the experimental conditions. The analysis showed strong correspondence between numerical and field data at medium and high deformation levels, as confirmed by the coefficient of determination <i>R</i>². Based on experimentally established material properties, a structural element with quasi-zero stiffness was modeled by combining negative-stiffness beams with positive-stiffness arches. Its force characteristics were investigated, and a working area with constant stiffness that effectively absorbs vibrations over a certain range was demonstrated. The results obtained confirm the feasibility of using TPU in the design of adaptive vibration isolation structures.</p>