Long-span spatial structures are typical city landmarks. Earthquakes can cause serious damage to these structures, leading to tremendous human injury and financial loss. Therefore, it is essential to develop effective devices to enhance the performance of spatial structures. This article proposes a new triple-tube glass fiber–reinforced polymer and steel buckling-restrained brace device for reticulated shells, which integrates the light weight and high strength advantages of the composite materials. Specimens of scaled glass fiber–reinforced polymer and steel buckling-restrained braces were designed and produced, and pseudo-static tests were performed on these specimens with an MTS machine. Mechanical performance and damages were examined and compared. An elaborate finite-element model was setup, and the accuracy of this model was verified with the test data. In addition, the model was used to investigate the effect of the Pe/Py ratio on the performance of full-scale triple-tube glass fiber–reinforced polymer and steel buckling-restrained brace devices. Finally, the lower limit of the Pe/Py ratio for this kind of buckling-restrained brace was obtained by theoretical derivation and numerical parametric analysis.