Recently, neuromorphic devices for artificial intelligence applications have attracted much attention. In this work, a three-terminal electrolyte-gated synaptic transistor based on NdNiO₃ epitaxial films, a typical correlated electron material, is presented. The voltage-controlled metal–insulator transition was achieved by inserting and extracting H⁺ ions in the NdNiO₃ channel through electrolyte gating. The non-volatile conductance change reached 10⁴ under a 2 V gate voltage. By manipulating the amount of inserted protons, the three-terminal NdNiO₃ artificial synapse imitated important synaptic functions, such as synaptic plasticity and spike-timing-dependent plasticity. These results show that the correlated material NdNiO₃ has great potential for applications in neuromorphic devices.