Thermoelectric energy conversion technology attracts great attention to convert the waste heat into electricity. For the practical thermoelectric application, figure of merit ZT > 1 (ZT = S²・sigma・T・kappa^-1, where Z, T, S, sigma, kappa are a figure of merit, the absolute temperature, Seebeck coefficient, electrical conductivity and thermal conductivity, respectively). Although heavy metal based materials such as Bi₂Te₃ and PbTe exhibit ZT ~ 1, they are not attractive, particularly operating at high temperatures (T~1,000K), because they are mostly toxic, and decomposition, vaporization or melting of the constituents can easily occur at high temperatures. This review provides a new class of thermoelectric material: SrTiO₃ having two-dimensional electron gas (2DEG). The high-density (~10²¹ cm^-3) 2DEG confined within the thickness of a SrTiO₃ unit cell layer (a = 0.3905 nm), which is realized at SrTiO₃/Nb-doped SrTiO₃ superlattices or TiO₂/SrTiO₃heterointerfaces. The 2DEG demonstrates that enhanced |S| by a factor of ~5 compared to that for bulk. And the optimized ZT for the 2DEG reaches 2.4, which is twice of conventional thermoelectric materials.