Two-dimensional　(2D)　Ruddlesden-Popper　hybrid　perovskites　(RPHPs)　have　shown　enormous　potential　for　photoelectric　applications　owing　to　their　advantages　of　structural　diversity　and　unique　physical　properties.　Despite　extensive　studies,　the　cage-incorporation　of　secondary　amines　(e.g.,　dimethylamine,　DMA)　to　explore　2D　RPHPs　remains　a　great　challenge.　Herein,　we　present　a　new　member　of　2D　RPHPs,　(BA)2(DMA)Pb2Br7(1,　BA+=n-butylammonium),　for　which　organic　DMA+cations　are　enclosed　in　perovskite　cavities.　The　inorganic　perovskite　sheets　and　organic　BA+spacing　layers　are　arranged　in　the　alternate　packings,　resembling　a　2D　quantum-well　motif.　This　feature　is　advantageous　for　the　photoconductivity　properties.　Consequently,　the　crystal-based　array　detectors　of1exhibit　strong　photodetection　performances,　including　high　detectivity　(2.0　×　1011Jones),　large　responsivity　(31.1　mA　W−1),　and　fast　response　time　(∼8　μs).　Moreover,　its　intrinsic　2D　quantum-well　structure　endows　significant　anisotropy　of　conductivity　and　optical　absorption,　which　account　for　the　strong　polarization　response　with　a　large　dichroic　ratio　of　∼1.43,　falling　in　the　range　of　other　2D　RPHPs.　These　results　reveal　the　potential　of1for　photodetection　and　simultaneously　shed　light　on　the　research　on　new　candidates　of　2D　RPHPs.　©　The　Royal　Society　of　Chemistry　2021.