Optical properties in a single crystal of the organic‐inorganic hybrid perovskite quantum‐well material (C4H9NH3)2(CH3NH3)Pb2Br7 have been investigated. This material has double‐layer sheets of corner‐sharing [PbBr6]4‐ octahedrons constituting the inorganic well. The estimated longitudinal‐transverse (LT) splitting, exciton exchange energy, and biexciton binding energy are about 60 meV, 17 meV and 26 meV respectively. We can also study well‐width dependence in angstroms by comparing with (C4H9NH3)2PbBr4 which has single‐layer sheets comprising the inorganic well. This is the first time the well‐width dependence of excitonic properties in hybrid organic‐inorganic quantum materials is investigated. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)