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Optoelectronic synaptic elements are emerging functional devices for the vigorous development of advanced neuromorphic computing technology in the post-Moore era. However, optoelectronic devices based on transition metal dichalcogenides (TMDs) are limited to their poor mobilities and weak light-matter interactions, which still hardly exhibit superior device performances in the application of artificial synapses. Here, we demonstrate the successful fabrication of Au nanoparticle-coupled MoS2 heterostructures via chemical vapor deposition (CVD), where the light absorption of MoS2 is greatly enhanced and engineered by plasmonic effects. Hot electrons are excited from Au nanoparticles, and then injected into MoS2 semiconductors under the light illumination. The plasmonically-engineered photo-gating effect at the metal-semiconductor junction is demonstrated to create optoelectronic devices with excellent synaptic behaviors, especially in ultra-sensitive excitatory postsynaptic current (EPSC, 9.6 × 10−3 nA@3.4 nW·cm−2), ultralow energy consumption (34.7 pJ), long-state retention time (> 1,000 s), and tunable synaptic plasticity transitions. The material system of Au-nanoparticles coupled TMDs presents unique advantages for building artificial synapses, which may lead the future development of neuromorphic electronics in optical information sensing and learning.