Abstract: Composite materials can play a decisive role to reveal novel physical properties and enable to advance new generation technologies. Here, we discover that phototransistors based on the integration of two-dimensional graphene nanosheets (GNSs) and amorphous indium–gallium–zinc–oxide (a-IGZO) semiconductors exhibit a giant photo-to-dark current ratio and long-lasting persistent photoconductivity (PPC). Under the illumination of UV light (350 nm) at 50 mW/cm2, a photo-to-dark current ratio up to 2.0 × 107 was obtained, which is about 3 orders of magnitude higher than its pure a-IGZO device counterpart. Moreover, the GNSs/a-IGZO phototransistor possesses an enduring lifetime up to years for the recovery of the transfer characteristics after switching off the UV light. The giant and long-lasting PPC leads GNSs/a-IGZO to become an excellent conductor with conductivity much better than indium tin oxide. The observed unique features represent a semiconductor–conductor transition. In addition to next generation flat, flexible, and display, it can open up a wide variety of application, such as transparent electrodes for optoelectronic devices, optical memory, and light harvesting for energy storage. As an example, we demonstrated the operation of optical memory devices, which may lead to the novel application of holographic storage. Our results shown here therefore provide an outstanding new route for the future development of solution-processable semiconducting optoelectronic devices.