AbstractOrganic–inorganic hybrid perovskites are exciting candidates for next-generation solar cells, with CH₃NH₃PbI₃ being one of the most widely studied. While there have been intense efforts to fabricate and optimize photovoltaic devices using CH₃NH₃PbI₃, critical questions remain regarding the crystal structure that governs its unique properties of the hybrid perovskite material. Here we report unambiguous evidence for crystallographic twin domains in tetragonal CH₃NH₃PbI₃, observed using low-dose transmission electron microscopy and selected area electron diffraction. The domains are around 100–300 nm wide, which disappear/reappear above/below the tetragonal-to-cubic phase transition temperature (approximate 57 °C) in a reversible process that often ‘memorizes’ the scale and orientation of the domains. Since these domains exist within the operational temperature range of solar cells, and have dimensions comparable to the thickness of typical CH₃NH₃PbI₃ films in the solar cells, understanding the twin geometry and orientation is essential for further improving perovskite solar cells.