The first-principles calculations are carried out to investigate the structural, elastic, electronic, optical, vibrational and thermodynamic properties of superhard diamond-like BC5 (d-BC5). The structural stability of BC5 is examined for previously proposed and several probable phases including F-43m, P6/mmm, Cmcm, Pnma, P-1, P3m1, Imm2, I-4m2 and Pmma. The most energetically stable phase is predicted to be Pmma. Computed bulk modulus B, shear modulus G, elastic constant C (44) and theoretical Vickers hardness H confirm that BC5 is an ultra-incompressible and superhard material. The electronic character analysis reveals the metallicity of BC5, indicating that a strong covalent bond network through sp (3) hybridization is the origin of its excellent mechanical properties. However, P-1 is found to be dynamically stable, contrary to the other study. Therefore, the phonon, thermodynamic and electronic properties of P-1 which are not available in the literature are discussed. The calculated physical parameters are in good agreement with the theoretical and experimental results. This work is expected to provide a useful guide for designing novel boride materials having superior mechanical performance.