It is of current interest to design sodium-ion batteries (SIBs) as one of the most promising alternatives to traditional lithium-ion batteries (LIBs). Motivated by the successful synthesis of atomic-thick BeN₄ sheets, going beyond the previously reported three-dimensional (3D) graphene- or silicene-monolith anode materials, we propose a new stable 3D monolith named oBeN₄ by assembling BeN₄ nanoribbons. A combined density functional theory computational study and tight-binding (TB) model analysis revealed that oBeN₄ is mechanically ductile and intrinsically metallic with conducting channels formed by electrons in the p-orbitals of N atoms. Light mass, high porosity, and intrinsic metallicity endow oBeN₄ with superior electrochemical performance when used as an anode material for SIBs, exhibiting a high reversible gravimetric capacity (578.03 mA h g⁻¹) and volumetric capacity (670.51 mA h cm⁻³), a small volume change (0.5%), low diffusion barriers (0.14–0.68 eV), and a low average open-circuit voltage (0.25 V). These findings demonstrate that the assembly of BeN₄ nanoribbons is a promising strategy for the design of novel SIB anodes with high performance.