Abstract: Isolation is an effective method for enhancing aseismic performance of structures and realizing seismic resilience. However, the poor tension-resistance capacity of conventional rubber bearings limited their application in buildings located in high seismic regions with large aspect ratios. Hence, it is necessary to develop a novel isolator to improve the tension-resistance capacity of conventional rubber bearings. This study proposed a tension-adjustable rubber isolator by introducing a tension-adjustable device (i.e., a friction damper) connected with the rubber bearing in series. The load-bearing and deformation capacities of the device were adjustable and controllable. Due to the series arrangement, the device uplifted in a limited and controllable manner to release the excessive tensile force, and transferred an adjustable tensile force below the design limit to the rubber bearing. The rubber bearing resisted this tensile force with its own tension-resistance capacity, ensuring a certain extent of overturning-resistance capacity of structures. Vertical compression/tension tests and horizontal compression-shear/tension-shear tests were conducted on both full-scaled conventional rubber bearing and the new tension-adjustable rubber isolator. Comparative analyses were performed on their compressive, tensile, and shear behaviors. The results indicated that the tension-adjustable device exhibited a negligible influence on the compressive behavior of rubber bearing, but it could successfully adjust the tensile force acting on the rubber bearing. By adjusting the preload of the friction damper, the activation force of the device could be accurately adjusted. In this research, the relative differences between the tested and theoretical activation forces were less than 3%, demonstrating the accurate adjustment of tensile force in the bearing. Under the compression-shear and tension-shear conditions, the behavior of the tension-adjustable rubber isolator and conventional rubber bearing were basically identical. Notably, the influence of vertical deformation of the tension-adjustable device on the shear behavior of the rubber bearing was also negligible. These findings indicated that the newly developed tension-adjustable isolator satisfied the dual requirements of horizontal isolation and vertical tension adjustments.