Materials possessing both superhard and metallic properties are beneficial for the creation of multifunctional devices under extreme conditions. Here, we report the formation of a new metallic superhard boron nitride at high pressure with stoichiometry B₂N₃ through first-principles calculations and structure searching. At ambient pressure, B₂N₃ has layered structures (h-B₂N₃) consisting of hexagonal B₄N₄ layers intercalated by triply bonded N₂ molecules. With the pressure increasing to ∼10 GPa, h-B₂N₃ transforms to a three-dimensional tetragonal structure (t-B₂N₃) with the formation of single N–N bonds. Calculations reveal that t-B₂N₃ can be recovered under ambient conditions in view of the dynamical, thermal and mechanical stability. Interestingly, t-B₂N₃ is proposed to be a superhard material with an estimated Vicker's hardness of ∼52 GPa by performing stress–strain calculations. More importantly, electronic calculations show unique two-dimensional metallicity in t-B₂N₃, which originates from the π orbitals of N–N bonds spreading in the ab plane. In addition, the energy density of ∼2.95 kJ g⁻¹ makes t-B₂N₃ a potential high-energy density material.