Segregation energies of impurity ions and oxygen vacancies at grain boundaries in Y2O3-doped ZrO2 as calculated from atomistic simulations using energy minimization and Monte Carlo methods are reported. Based on these energies, local defect equilibrium concentrations have been estimated. It is found that it is more energetically favorable for an yttrium ion to be accompanied by an oxygen vacancy at grain boundaries, although decrease in energy when associated with an oxygen vacancy differs from boundary to boundary. The segregation energy for a neutral defect complex consisting of a two yttrium ions and an oxygen vacancy at infinitely dilute concentration is highly correlated with the coordination environment of each site in the vicinity of the grain boundary (GB), and, in turn, GB energy. Although the estimated local equilibrium concentrations of these defects are similar, detailed analysis of the atomic coordination and defect distributions in the vicinity of a GB reveal that defect distributions, especially of oxygen vacancies, are dependent on the characteristics of the particular GB and that segregation in effect reduces lattice strains at the GB. Equilibrium concentration distributions of yttrium at grain boundaries are also given as a function of spatial resolution, and are useful for interpretation of experimental results.