A photomultiplication (PM)‐type organic photodetector (OPD) that exploits the ionic motion in CsPbI3 perovskite quantum dots (QDs) is demonstrated. The device uses a QD monolayer as a PM‐inducing interlayer and a donor–acceptor bulk heterojunction (BHJ) layer as a photoactive layer. When the device is illuminated, negative ions in the CsPbI3 QD migrate and accumulate near the interface between the QDs and the electrode; these processes induce hole injection from the electrode and yield the PM phenomenon with an external quantum efficiency (EQE) >2000% at a 3 V applied bias. It is confirmed that the ionic motion of the CsPbI3 QDs can induce a shift in the work function of the QD/electrode interface and that the dynamics of ionic motion determines the response speed of the device. The PM OPD showed a large EQE‐bandwidth product >106 Hz with a −3 dB frequency of 125 kHz at 3 V, which is one of the highest response speeds reported for a PM OPD. The PM‐inducing strategy that exploits ionic motion of the interlayer is a potential approach to achieving high‐efficiency PM OPDs. A photomultiplication‐type organic photodetector that exploits the ion migration in perovskite quantum dot interlayer is demonstrated. Under illumination, negative ions in the quantum dot migrate and accumulate near the quantum dot/electrode interface inducing photomultiplication which results in large external quantum efficiency and bandwidth. The relation between the bandwidth and dynamics of the ionic motion is also discussed.