In this paper, a physical identification procedure considering the torsionally coupled effect is developed to investigate the dynamic characteristics of an asymmetric base-isolation building equipped with lead–rubber bearings. A rigid superstructure is assumed to approximate the dynamic characteristics of a squat base-isolated structure. The torsional stiffness of the isolation system is considered linear, whereas the translational stiffness in both the x and y directions is assumed bilinear. The hysteresis of the base shear in relation to the bearing displacement is characterized by a backbone curve, by which the multivalued force–deformation relationship can be transformed into a single-valued function, thus simplifying the system identification task. The proposed algorithm extracts the physical parameters of the isolation system in the three independent directions, thereby providing critical information for structural health monitoring. A numerical example is used to demonstrate the feasibility of the proposed technique for asymmetric base-isolated buildings.