Abstract Different from conventional insulating or semiconducting boron nitride, metallic BN has received increasing attention in recent years as its intrinsic metallicity grants it great potential for broad applications. In this study, by assembling the experimentally synthesized pentagonal B₂N₃ units, we have proposed the first pentagon-based three-dimensional (3D) metallic boron nitride, labeled penta-B₄N_7. First-principles calculations together with molecular dynamics simulations and convex hull diagram show that penta-B₄N₇ is not only thermally, dynamically and mechanically stable, but also three dimensionally metallic. A detailed analysis of its electronic structure reveals that the intrinsic metallicity comes from the delocalized electrons in the partially occupied antibonding N–N π orbitals. Equally important, the energy density of penta-B₄N₇ is found to be 4.07 kJ g⁻¹, which is the highest among that of all the 3D boron nitrides reported so far.