Enhancement of the Seebeck coeffcient (S) without reducing the electrical conductivity (sigma) is essential to realize practical thermoelectric materials exhibiting a dimensionless figure of merit (ZT = S-2 (.) s (.) T (.) kappa(-1)) exceeding 2, where T is the absolute temperature and. is the thermal conductivity. Here, we demonstrate that a high-density two-dimensional electron gas (2DEG) confined within a unit cell layer thickness in SrTiO3 yields unusually large vertical bar S vertical bar, approximately five times larger than that of SrTiO3 bulks, while maintaining a high sigma(2DEG). In the best case, we observe vertical bar S vertical bar = 850 mu VK-1 and sigma(2DEG) = 1.4 x 10(3) S cm(-1). In addition, by using the kappa of bulk single-crystal SrTiO3 at room temperature, we estimate ZT similar to 2.4 for the 2DEG, corresponding to ZT similar to 0.24 for a complete device having the 2DEG as the active region. The present approach using a 2DEG provides a new route to realize practical thermoelectric materials without the use of toxic heavy elements.