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Reversible antiferroelectric‐ferroelectric phase transitions were recently observed in a series of SrSnO₃‐modified NaNbO₃ lead‐free antiferroelectric materials, exhibiting well‐defined double polarization hysteresis loops at ambient conditions. Here, transmission electron microscopy was employed to investigate the crystallography and domain configuration of this newly designed system via electron diffraction and centered dark‐field imaging. It was confirmed that antiferroelectricity is maintained in all compositions, manifested by the characteristic ¼ superlattice reflections in the electron‐diffraction patterns. By investigating the antiferroelectric domains and domain boundaries in NaNbO₃, we demonstrate that antiphase boundaries are present and their irregular periodicity is responsible for the streaking features along the ¼ superlattice reflections in the electron‐diffraction patterns. The signature domain blocks observed in pure NaNbO₃ are maintained in the SrSnO₃‐modified ceramics, but disappear when the amount of SrSnO₃ reaches 7 mol.%. In particular, a well‐defined and distinct domain configuration is observed in the NaNbO₃ sample modified with 5 mol.% SrSnO₃, which presents a parallelogram domain morphology.