2D　organic–inorganic　hybrid　perovskites　are　emerging　as　the　promising　alternatives　in　photodetection,　due　to　their　unique　merits　stemming　from　intrinsic　quantum-confined　structures.　Despite　great　ongoing　effects,　highly　sensitive　and　ultrafast　responding　photodetectors　of　2D　hybrid　perovskites　remain　a　huge　blank.　Here,　the　first　nanosecond-responsive　array　photodetector　based　on　2D　perovskite　crystals　of　(PA)2(G)Pb2I7　(where　PA　=　n-pentylaminium　and　G　=　guanidinium)　is　fabricated,　which　features　a　newly　tailored　bilayer　motif　with　large-size　G　cation　as　“perovskiter”　inside　the　distorted　PbI3　perovskite　framework.　This　motif　creates　a　new　branch　of　the　intriguing　2D　hybrid　perovskite　family.　Strikingly,　both　high　photodetectivities　(6.3　×　1012　Jones)　and　responsivities　(≈47　A　W−1),　as　well　as　low　dark　current　(≈2.4　×　10−11　A),　are　achieved　in　this　array　photodetector.　Further,　its　ultrafast　response　time　(τ)　of　3.1　ns　is　one　of　the　record-fast　merits　for　2D　hybrid　perovskites.　Such　attributes　originate　from　its　strong　in-plane　photoresponse　related　to　perovskite　network　and　low　dark　current　inhibited　by　the　hopping　barriers　of　insulating　PA　bilayers.　All　these　characteristics　make　(PA)2(G)Pb2I7　a　promising　candidate　for　photodetection,　and　pave　the　way　to　explore　new　2D　hybrid　perovskite　candidates　for　future　high-performance　optoelectronic　devices.　©　2019　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim