The ground and low-lying electronic states of ReB were studied at the CASPT2//CASSCF level (multiconfigurational second-order perturbation theory) and quadruple-ζ ANO-RCC basis sets. Spectroscopic constants, potential energy curves, wavefunctions, and Mulliken population analysis are given. The ground state of ReB is of X(5)Σ(+) symmetry (R e  = 1.817 Å, ω e  = .909 cm(-1), and μ = 2.87 D), giving rise to a Ω = 0(+) ground state after including spin-orbit coupling.