The structure of the cubic-ZrO2 symmetrical tilt Sigma5 (310)/ [001] grain boundary is examined using density functional theory within the local density and pseudopotential approximations. Several pristine stoichiometric grain-boundary structures are investigated and compared with Z-contrast scanning transmission electron microscopy and electron energy loss spectroscopy results. The lowest-energy grain-boundary structure is found to agree well with the experimental data. When Y3+ is substituted for Zr4+ at various sites in the lowest-energy grain-boundary structure, the calculations indicate that Y3+ segregation to the grain boundary is energetically preferred to bulk doping, in agreement with experimental results.