CsPbI3-based perovskite solar cells (PSCs) have attracted intense research interest since the inorganic absorber layer has better thermal stability compared with hybrid perovskites. However, CsPbI3 suffers from structural instability due to an easily induced phase transition from the photoactive phase to the photoinactive phase. Introducing (CH3)(2)NH2I (dimethylammonium iodide [DMAI]) into the CsPbI3 precursor solution stabilizes the tetragonal (beta-phase) of CsPbI3, but there is still debate about whether DMA(+) is incorporated in the perovskite structure. Here, we report unambiguous evidence for the formation of tetragonal (beta-) (DMA, Cs)PbI3 by substituting the small ionic radius Cs+ with the large organic cation DMA(+). The organic-inorganic hybrid beta-(DMA, Cs)PbI3 shows a better optoelectronic properties than inorganic orthorhombic (gamma-phase) CsPbI3. Consequently, PSCs based on beta-(DMA, Cs)PbI3 exhibit a champion power conversion efficiency of 19.76%. These observations suggest that hybrid beta-(DMA, Cs)PbI3, with a dominant inorganic composition, is preferable compared with inorganic gamma-CsPbI3 for efficient and stable PSCs.