The atomic surface structure of SrTiO ₃(100) after annealing at 630 ° C in vacuum is investigated by x-ray photoelectron diffraction (XPD) using the Sr  3d_5/2 core level. The photoelectron diffraction peaks are successfully assigned by considering the forward scattering of photoelectrons by the atomic potential near the emitter atom in the lattice. The strongest diffraction peaks are aligned along the single crystal internuclear axes. We compare the results of photoelectron multiple scattering calculations (MSC) of SrO and TiO ₂ terminated SrTiO ₃(100) surfaces, including surface relaxation and rumpling, with the experimental data. For TiO ₂ and SrO terminated SrTiO ₃(100) surfaces, all top-layer cations relax inward, whereas second-layer atoms relax outward. The surface rumpling for SrO- and TiO ₂ -terminated surfaces agrees well with low-energy electron diffraction results. Using a genetic algorithm the best agreement of MSC to the experimental XPD data is obtained for a SrO terminated surface with a 30% coverage of 3 ML SrO(100) islands.