Hybrid systems have recently attracted increasing attention, which combine the special attributes of each constitute and create interesting functionalities through multiple heterointerface interactions. Here, we design a two-dimensional (2D) hybrid phototransistor utilizing Janus-interface engineering, in which the WSe 2 channel combines light-sensitive perovskite and spontaneously polarized ferroelectrics, achieving collective ultrasensitive detection performance. The top perovskite (BA 2 (MA) 3 Pb 4 I 13 ) layer can absorb the light efficiently and provide generous photoexcited holes to WSe 2 . WSe 2 exhibit p-type semiconducting states of different degrees due to the selective light-operated doping effect, which also enables the ultrahigh photocurrent of the device. The bottom ferroelectric (Hf 0.5 Zr 0.5 O 2 ) layer dramatically decreases the dark current, which should be attributed to the ferroelectric polarization assisted charge trapping effect and improved gate control. As a whole, our phototransistors show excellent photoelectric performances across the ultraviolet to near-infrared range (360−1050 nm), including an ultrahigh ON/OFF current ratio > 10 9 and low noise-equivalent power of 1.3 fW/Hz 1/2 , all of which are highly competitive in 2D semiconductor-based optoelectronic devices. In particular, the devices show excellent weak light detection ability, where the distinguishable photoswitching signal is obtained even under a record-low light intensity down to 1.6 nW/cm 2 , while showing a high responsivity of 2.3 × 10 5 A/W and a specific detectivity of 4.1 × 10 14 Jones. Our work demonstrates that Janus-interface design makes the upper and lower interfaces complement each other for the joint advancement into high-performance optoelectronic applications, providing a picture to realize the integrated engineering on carrier dynamics by light irradiation, electric field, interfacial trapping, and band alignment.