Flash sintering of strontium titanate (SrTiO3) is studied at different applied fields to understand its effect on density and grain growth. In particular, the defect structure is investigated by optical and structural analysis. SrTiO3 exhibited a trend in densification opposite that of ionically or electronically conductive ceramics: as the applied voltage decreased, the density increased. Abnormal grain growth in conventionally sintered SrTiO3 is arrested by flash sintering. Interestingly, undoped SrTiO3 behaved differently than undoped Al2O3, which did not exhibit any signs of flash sintering. Previous attempts at flash sintering could only be achieved in MgO-doped Al2O3. We believe that non-stoichiometric Ruddlesden-Popper phases in SrTiO3, as indicated by ultrafast optical spectroscopy, X-ray diffraction, conductivity measurements, and transmission electron microscopy, assist flash sintering by increasing local conductivity through enhanced defect content.