Highly responsive organic image sensors are crucial for medical imaging applications. To enhance the pixelwise photoresponse in an organic image sensor, the integration of an organic photodetector with amplifiers, or the use of a highly responsive organic photodetector without an additional amplifying component, is required. The use of vertically stacked, two‐terminal organic photodetectors with photomultiplication is a promising approach for highly responsive organic image sensors owing to their simple two‐terminal structure and intrinsically large responsivity. However, there are no demonstrations of an imaging sensor array using organic photomultiplication photodetectors. The main obstacle to a sensor array is the weak‐light sensitivity, which is limited by a relatively large dark current. Herein, a highly responsive organic image sensor based on monolithic, vertically stacked two‐terminal pixels is presented. This is achieved using pixels of a vertically stacked diode‐type organic photodetector with photomultiplication. Furthermore, applying an optimized injection electrode and additionally stacked rectifying layers, this two‐terminal device simultaneously demonstrates a high responsivity (>40 A W−1), low dark current, and high rectification under illumination. An organic image sensor based on this device with an extremely simple architecture exhibits a high pixel photoresponse, demonstrating a weak‐light imaging capability even at 1 µW cm−2. A highly responsive organic image sensor based on vertically stacked two‐terminal pixels is achieved with pixels of a diode‐type organic photodetector through photomultiplication. With an optimized injection electrode and additionally stacked rectifying layers, the organic image sensor with an extremely simple architecture exhibits a high pixel photoresponse and demonstrates a weak‐light imaging capability at 1 µW cm−2.