Anomalous　negative　phototransistors　in　which　the　channel　current　decreases　under　light　illumination　hold　potential　to　generate　novel　and　multifunctional　optoelectronic　applications.　Although　a　variety　of　design　strategies　have　been　developed　to　construct　such　devices,　NPTs　still　suffer　from　far　lower　device　performance　compared　to　well-developed　positive　phototransistors　(PPTs).　In　this　work,　a　novel　1D/2D　molecular　crystal　p–n　heterojunction,　in　which　p-type　1D　molecular　crystal　(1DMC)　arrays　are　embedded　into　n-type　2D　molecular　crystals　(2DMCs),　is　developed　to　produce　ultrasensitive　NPTs.　The　p-type　1DMC　arrays　act　as　light-absorbing　layers　to　induce　p-doping　of　n-type　2DMCs　through　charge　transfer　under　illumination,　resulting　in　ineffective　gate　control　and　significant　negative　photoresponses.　As　a　result,　the　NPTs　show　remarkable　performances　in　photoresponsivity　(P)　(1.9 × 108)　and　detectivity　(D*)　(1.7 × 1017　Jones),　greatly　outperforming　previously　reported　NPTs,　which　are　one　of　the　highest　values　among　all　organic　phototransistors.　Moreover,　the　device　exhibits　intriguing　characteristics　undiscovered　in　PPTs,　including　precise　control　of　the　threshold　voltage　by　controlling　light　signals　and　ultrasensitive　detection　of　weak　light.　As　a　proof-of-concept,　the　NTPs　are　demonstrated　as　light　encoders　that　can　encrypt　electrical　signals　by　light.　These　findings　represent　a　milestone　for　negative　phototransistors,　and　pave　the　way　for　the　development　of　future　novel　optoelectronic　applications.　©　2022　Wiley-VCH　GmbH.