Many-body interactions in transition-metal oxides give rise to a wide range of functional properties, such as high-temperature superconductivity1, colossal magnetoresistance2 or multiferroicity 3. The seminal recent discovery of a two-dimensional electron gas (2DEG) at the interface of the insulating oxides LaAlO3 and SrTiO3 (ref. 4) represents an important milestone towards exploiting such properties in all-oxide devices5. This conducting interface shows a number of appealing properties, including a high electron mobility4, 6, superconductivity7 and large magnetoresistance8, and can be patterned on the few-nanometre length scale. However, the microscopic origin of the interface 2DEG is poorly understood. Here, we show that a similar 2DEG, with an electron density as large as 8×1013 cm−2, can be formed at the bare SrTiO3 surface. Furthermore, we find that the 2DEG density can be controlled through exposure of the surface to intense ultraviolet light. Subsequent angle-resolved photoemission spectroscopy measurements reveal an unusual coexistence of a light quasiparticle mass and signatures of strong many-body interactions.