Photodetectors and optoelectronic synapses are vital for construction of artificial visual perception system. However, the hardware implementations of optoelectronic-neuromorphic devices based on conventional architecture usually suffer from poor scalability, light response range, and limited functionalities. Here, large-scale flexible monolayer MoS₂ devices integrating photodetectors and optoelectronic synapses over the entire visible spectrum in one device have been realized, which can be used in photodetection, optical communication, artificial visual perception system, and optical artificial neural network. By modulating gate voltages, we enable MoS₂-based devices to be photodetectors and also optoelectronic synapses. Importantly, the MoS₂-based optoelectronic synapses could implement many synaptic functions and neuromorphic characteristics, such as short-term memory (STM), long-term memory (LTM), paired-pulse facilitation (PPF), long-term potentiation (LTP)/long-term depression (LTD), and “learning-experience” behavior. Furthermore, an associative learning behavior (the classical conditioning Pavlov’s dog experiment) was emulated using paired stimulation of optical and voltage pulses. These results facilitate the development of MoS₂-based multifunctional optoelectronic devices with a simple device structure, showing great potential for photodetection, optoelectronic neuromorphic computing, human visual systems mimicking, as well as wearable and implantable electronics.