The electronic structure of the ruthenium blue dimer was studied with hybrid density functional theory (DFT) and complete action space self-consistent field (CASSCF) calculations. Hybrid DFT was found to overestimate the stability of the lowest triplet which leads to predicting this state as the ground state. However, CASSCF gives a singlet ground state in agreement with experiment. The density of the CASSCF ground state is in good agreement with the broken symmetry state obtained from DFT. These two results lead to the interpretation that the ground state of the ruthenium blue dimer is a partially localized antiferromagnetically coupled singlet state that lies three-quarters of the way toward the weak coupling limit. The electrons were found to localize in the d(pi) manifold.