We investigate structural, electronic, and ferroelectric properties of A-cation ordered perovskite BaSrBi22O66 using first-principles density functional theory. We systematically search for the metastable structures by constructing the initial structures with A-site orderings and octahedral rotation patterns commonly observed in perovskite oxides. In all of the metastable structures, we find that the breathing distortions observed in the bulk BaBiO33 and SrBiO33 are maintained, which stabilizes the charge-ordered insulating phase. More importantly, we identify a polar ground state with layered A-cation ordering and a−a−c+ octahedral rotation pattern with a net polarization of 5.7 μC/cm22. Our distortion mode analysis shows that the polarization is induced from the hybrid improper ferroelectricity in which the polar A-site displacements from Ba/Sr layered ordering emerge from the coupling to the octahedral rotations. We believe that our method to identify the metastable structures can be used in a wide range of perovskite oxides and the proposed ferroelectric BaSrBi22O66 can be fabricated by layer-by-layer growth techniques.