Semiconductor-based sensors have played an important role in efficient detection of combustible, flammable, and toxic gases, but they usually need to operate at elevated temperatures (200 degrees C or higher). Although reducing the operation temperature down to room temperature is of practical significance, it is still a huge challenge to fabricate room temperature sensors with a low cost. Here we show a novel "self-doping" strategy to overcome simultaneously both difficulties of "high resistance" and "low reaction rate", which have always been encountered for room-temperature operation of semiconductor-based sensors. In particular, a porous crystalline titania with heavily self-doped Ti3+ species has been prepared by using a porous amorphous TiO2 and urea as the starting materials. The resulting Ti3+ self-doped TiO2 material serves as an efficient room-temperature gas-sensing material for specific CO detection with fast response/recovery. The self-dopant (Ti3+) in the titania material has proved to decrease the resistance of TiO2 significantly on the one hand and to increase the chemisorbed oxygen species substantially, thus enhancing the surface reaction activity on the other. Such a self-doping concept is anticipated to give a fresh impetus to rational design of room-temperature sensing devices with low costs.