Synaptic devices inspired by biological systems have the potential to overcome the limitations of traditional computers based on von Neumann architecture. There have been attempts to use GaN as a synaptic device, but it is challenging due to the short decay time caused by rapid recombination, despite the presence of trap sites within the GaN. In this study, we present a graphene/GaN optoelectronic synaptic device that controls current weights using ultraviolet light. Through Photoluminescence measurements, we reveal that graphene can promote efficient separation of electron-hole pairs in GaN, which in turn suppresses recombination and extends the carrier decay time. Building on this mechanism, our results show that the graphene/GaN optoelectronic synaptic devices are capable of not only emulating a range of biologically inspired synaptic behaviors but also memorizing letter patterns.