Polarized-light　photodetectors　are　the　indispensable　elements　for　practical　optical　and　optoelectronic　device　applications.　Two-dimensional　(2D)　hybrid　perovskite　ferroelectrics,　in　which　the　coupling　of　spontaneous　polarization　(Ps)　and　light　favors　the　dissociation　of　photo-induced　carriers,　have　taken　a　booming　position　within　this　portfolio.　However,　polarized-light　photodetectors　with　a　low　detection-limit　remain　unexplored　in　this　2D　ferroelectric　family.　In　this　work,　the　high-quality　individual　crystals　of　a　2D　perovskite　ferroelectric,　BA2CsPb2Br7　(1,　where　BA+　is　n-butylammonium),　were　used　to　fabricate　ultrasensitive　polarized-light　detectors.　Its　unique　bilayered　structural　motif　results　in　quite　strong　electric　and　optical　anisotropy　with　a　large　absorption　ratio　of　αc/αa　≈　3.2　(λ　=　405　nm).　Besides,　the　presence　of　ferroelectric　Ps　also　endows　high　built-in　electric　field　along　the　polar　c-axis　that　favors　photoelectric　activities.　Under　an　extremely　low　detectable　limit　of　40　nW/cm2,　the　detector　of　1　exhibits　a　notable　dichroism　ratio　(Iphc/Ipha　≈　1.5),　a　large　responsivity　of　~39.5　mA/W　and　a　specific　detectivity　of　~1.2　×　1012　Jones.　Moreover,　crystal-based　devices　of　1　also　exhibit　a　fast　response　speed　(~300　μs)　and　excellent　anti-fatigue　merits.　This　work　highlights　great　potentials　of　hybrid　perovskite　ferroelectrics　toward　polarized-light　photodetection.　©　2020　Science　China　Press