Graphene is an attractive optoelectronic material for light detection because of its broadband light absorption and fast response time. However, the relatively low absorption cross-section, fast recombination rate, and the absence of gain mechanism have limited the responsivity of pure graphene-based phototransistor to ≈10−2 A W−1. In this work, a photoconductive gain of ≈109 electrons per photon and a responsivity of ≈6.0 × 105 A W−1 are demonstrated in a hybrid photodetector that consists of monolayer graphene covered with a thin layer of dispersive organolead halide perovskite (CH3NH3PbBr2I) islands. The unprecedented performance is attributed to the effective charge transfer and photogating effect, which were evidenced by photoluminescence quenching, time-resolved photoluminescence decay, scanning near-field optical microscopy, and photocurrent mapping. Unlike previous report which used perovskite bulk thin film, the perovskite islands have low bulk recombination rate of photogenerated carriers. The device also shows broad photodetection spectral range from ultraviolet to visible (250–700 nm), affording new opportunities for scalable UV detectors and imaging sensors.