A thermoelectric device including: a thermoelectric material layer comprising a thermoelectric material; a transition layer on the thermoelectric material; and a diffusion prevention layer on the transition layer, wherein the thermoelectric material comprises a compound of Formula 1: (A1-aA′a)4-x(B1-bB′b)3-y-zCz  Formula 1 wherein A and A′ are different from each other, A is a Group 13 element, and A′ is at least one element of a Group 13 element, a Group 14 element, a rare-earth element, or a transition metal, B and B′ are different from each other, B is a Group 16 element, and B′ is at least one element of a Group 14 element, a Group 15 element, or a Group 16 element, C is at least one halogen atom, a complies with the inequality 0≤a