Recent shock compression experiments produced clear evidence of a new carbon phase, but a full structural identification has remained elusive. Here we establish by ab initio calculations a body-centered cubic carbon phase in Ia3¯d(O10h) symmetry, which contains twelve atoms in its primitive cell, thus termed BC12, and comprises all-sp3 six-membered rings. This structural configuration places BC12 carbon in the same bonding type as cubic diamond, and its stability is verified by phonon mode analysis. Simulated x-ray diffraction patterns provide an excellent match to the previously unexplained distinct diffraction peak found in shock compression experiments. Electronic band and density of states calculations reveal that BC12 is a semiconductor with a direct band gap of ∼2.97eV. These results provide a solid foundation for further exploration of this new carbon allotrope.