Very recently boron/nitrogen codoped titanium dioxide (B/N@TiO2) has been synthesized and the crystals are red in color with excellent absorption over the full visible spectrum. In this report, the geometries and electronic states of B/N@TiO2 have been investigated using density functional theory plus Hubbard U contributions. It is found that (i) the net charges introduced by the dopants can be fully compensated when the B/N ratio is 1/3; (ii) when B-doping and B/N-codoping coexist, the electronic states associated with unbalanced B-dopants can form impurity bands; and (iii) the electron excitation from the impurity bands to conduction bands can lead to the optical absorption at ∼2.0 eV, which has been proposed as the origin of the excellent absorption of visible light offered by red TiO2.