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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 …