A neuromorphic device includes a synapse. The synapse, according to an embodiment, 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 reactive metal layer disposed between the oxygen-containing layer and the second electrode. The oxygen-containing layer includes oxygen ions. The reactive metal layer is capable of reacting with the oxygen ions of the oxygen-containing layer. A width of the reactive metal layer decreases along a direction toward the oxygen-containing layer from the second electrode.