From crystal chemistry rationale and geometry optimization onto ground state structures with DFT-based computations, a novel ultra-hard body centered tetragonal C 4 (tet-C 4) is proposed as the simplest dense carbon allotrope. tet-C 4 is identified as cohesive as diamond, and likewise built of corner sharing C4 tetrahedra in all directions. Qualified as mechanically stable from the elastic constants combinations, tet-C 4 has Vickers hardness identical to that of both cubic and hexagonal (lonsdaleite) diamond. Dynamical stability of tet-C 4 derived from all positive phonon bands also allows obtaining heat capacity close to the experimental data of diamond. The electronic band tructure shows insulating tet-C 4 with a large indirect band gap of 5 eV.