Inspired by the human brain, neuromorphic computing has recently attracted much research attention. However, majority of the current research efforts towards developing artificial synapses are based on the binary SRAMs, making it impractical to scale up the system to the level of complexity we need. We present a novel approach to build electrochemically-tunable, two-dimensional (2D) synapses with excellent controllability, good energy efficiency, symmetric resistance response, and a rare combination of low-power programming and good retention. In our 2D synapses, the channel conductance (synaptic weight) can be modulated by controlling the concentration of ions between layers of 2D materials through a process called electrochemical intercalation. The major advantage is that we can achieve reversible and precise programming of the 2D device’s conductance to mimic synaptic plasticity with low power consumption. This work can lead to the low-power hardware implementation of neural networks for neuromorphic computing.