© 2020 The Authors While buckling-restrained braces offer excellent energy dissipation characteristics, their low post-yield stiffness may result in large residual drifts and interstory drift concentration when used in simply supported frames. This paper introduces a new multistage buckling-restrained brace to help mitigate these design challenges. The proposed device features two low yield point (LYP) cores with LY225 and short yield lengths, and one high yield point (HYP) core with SA440B and a longer yield length. In a design level event, the LYP cores dissipate energy, while the parallel HYP core provides an elastic restoring force. At large drifts, the HYP core yields and the device acts similar to a high-capacity, ductile, conventional BRB. A 384 kN specimen was tested at up to 1.5% strain and the individual core contributions recorded using strain gauges attached to the elastic core segments inside the restrainer. The multistage response matched the predicted trilinear backbone, achieving 10 to 20% equivalent damping prior to yielding the HYP core, and a fatigue capacity exceeding three times the AISC 341-16 acceptance criteria. Interaction between the decoupled cores was studied using a 3D finite element model, indicating that minor detailing changes could further improve performance.