Deep impurity states associated with a substitutional nitrogen at an oxygen site (N(O)) are believed to be the source of the visible-light absorption of nitrogen-doped titanium dioxide (TiO(2)). Our comprehensive study using density functional theory (DFT) plus onsite Coulomb interaction (U) reveals that a titanium atom at an interstitial site (Ti(i)) is highly mobile and strongly binds with N(O). Hybridizations of N p with Ti d states of Ti(i) give rise to a new band at the valence band edge, eliminating the hole-trapping centers originated from the deep N(O) states. The suggested mechanism explains the photocatalytic oxidation reactions as well as the visible-light absorption observed on N-doped anatase TiO(2).