Abstract: This paper presents the design of a displacement-type metal damper that utilizes torsional yield energy dissipation. The damper features a diamond-shaped configuration, employing a spatially asymmetric structure to achieve pure torsional energy dissipation through a single circular tube. When relative linear displacements occur at both ends of the damper, equal and opposite torques are generated at the extremities of the metal tube; once it enters the plastic deformation stage, energy dissipation commences, resulting in corresponding damping forces at both ends. The fundamental mechanical properties of this damper are thoroughly analyzed, and theoretical expressions for initial stiffness and yield load are derived based on bilinear elasto-plastic assumptions while accounting for linkage structures and connection stiffness effects. An analysis of geometric large deformations is conducted to evaluate the lateral unbalance coefficient of the damper, demonstrating its capability to maintain equilibrium over an extensive range. A prototype diamond-shaped metal damper with a yield load capacity of 16 tons has been manufactured according to these theoretical expressions and is designed for displacements up to ±40 mm. Its mechanical performance and fatigue characteristics are assessed through cyclic loading tests. A comparison among experimental skeleton curves, finite element simulation results based on Chaboche kinematic hardening model and theoretically derived skeleton curves indicates that our established theoretical model aligns well with both simulation outcomes and experimental data, thereby providing valuable insights for future designs. Fatigue testing involving 60 cycles at a ductility factor of 12.5 reveals consistently full hysteresis loops without any damage occurring in the damper—indicating exceptional fatigue performance. We derive relationships between designed shear strain parameters such as torsion tube length and inner-to-outer diameter ratios under specified yield loads. This leads us to propose a decoupled design process for displacement-type metal dampers characterized by torsional yield energy dissipation—first focusing on yielding design followed by considerations related to fatigue—which offers significant advantages over traditional bending or axial yielding dampers.