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The all-inorganic halide perovskite CsPbBr3has attracted significant attention for applications of high-performance and stable solar cells, light-emitting diodes (LEDs), and photoelectric and X-/γ-ray detectors. The discrepant photoelectric properties are confusing, which strongly implies that the perovskite is sensitive to defects. Here, novel and ubiquitous defects, “twin domains”, are reported in solution-grown CsPbBr3single crystals on the basis of X-ray diffraction techniques, optical microscopy, and transmission electron microscopy (TEM). Two kinds of orthorhombic domain structures, {121} reflection twins and 90° rotations along [101̅], are identified in as-grown CsPbBr3crystals. In situTEM results indicate that these domains are ferroelastic, which are related to the spontaneous strain induced by a phase transition. During a phase transition from parent cubic (m3̅m) to tetragonal (4/mmm), the loss of cubic {110} reflection planes, the 4-fold rotation axis (4), or the 3-fold rotoinversion axis (3̅) could act as a twin law and account for the appearance of domain walls. Further reduction of symmetry to orthorhombic (mmm) and the losses of the tetragonal 4-fold rotation axis (4) and mirror plane {101} lead to the emergence of [101̅] rotations and {101} reflection twins, respectively. Our work not only enhances the understanding of defects in CsPbBr3, which opens the door for the further optimization of photoelectric applications, but also provides a way to tailor it.