A neuromorphic device includes a synapse. The synapse includes a first electrode, a second electrode spaced apart from the first electrode, an oxygen-containing layer disposed between the first electrode and the second electrode, the oxygen-containing layer including oxygen ions, and a stack structure disposed between the oxygen-containing layer and the second electrode, the stack structure including a plurality of reactive metal layers alternately arranged with a plurality of oxygen diffusion-retarding layers. The plurality of reactive metal layers are capable of reacting with oxygen ions of the oxygen-containing layer. The plurality of oxygen diffusion-retarding layers interfere with a movement of the oxygen ions from the oxygen-containing layer to the plurality of reactive metal layers.