Provided are a dielectric including an oxide represented by Formula 1 below and having a cubic crystal structure, a capacitor including the dielectric, a semiconductor device including the dielectric, and a method of manufacturing the dielectric. (RbxA1-x)(ByTa1-y)O3-δ   In Formula 1 above, A is K, Na, Li, Cs, or a combination thereof, B is Nb, V, or a combination thereof, and 0.1≤x≤0.2, 0≤y≤0.2, and 0≤δ≤0.5 are satisfied.