A p-type phototransistor based on tin monoxide (SnO) thin film and a perovskite-mediated photogating effect to enhance the device performance are reported. Without the perovskite layer, the SnO thin film phototransistor exhibits a good figure of merit including on/off ratio (Ion/Ioff) over 103, hole mobility of 3.55 cm2 V−1 s−1, photoresponsivity of 1.83 × 103 A W−1, and detectivity of 2.11 × 1013 Jones at 655 nm; these values are among the highest reported for oxide-based devices. Furthermore, it is shown that when a hybrid perovskite MAPbI3 overlayer is deposited on the p-type SnO channel, the phototransistor behavior is significantly modified. Specifically, the field-effect mobility increases to 5.53 cm2 V−1 s−1 and the on/off ratio increases to 2.7 × 103 compared with an on/off ratio of 519 under dark. These effects can be ascribed to a perovskite-mediated photogating effect with favorable band alignment and interfacial charge transfer. This work not only introduces SnO as a new p-type phototransistor material but also presents a general approach to enhance the channel transport via coating a photoactive perovskite overlayer.