Nitrogen/fluorine codoping of rutile TiO2 was recently reported to be effective for introducing visible-light absorption, and the resultant TiO2:N,F worked efficiently as an O-2 evolution photocatalyst in a Z-scheme water-splitting system. Although an increase in the amount of nitrogen doped into rutile TiO2 lattice in the presence of fluorine was experimentally demonstrated, the role of fluorine in the system remained unclear. Here, we report a computational study on TiO2:N,F through the construction of supercell models with substitutional defects to reveal the atomic arrangement of the material and the electronic band structure. Calculations for all possible structures of nitrogen/ fluorine and nitrogen/oxygen-vacancy relative positions revealed that the defect complexes were preferentially located on the (110) plane and that the distance between defects did not have a strong correlation with the formation energy. The present work also showed that although fluorine did not directly contribute to the narrowing of the band gap of TiO2:N,F, the fluorine activity of the synthetic atmosphere promotes the formation of substitutional defect complexes of nitrogen/fluorine for anion sites. This eventually increases the amount of nitrogen incorporated into the rutile TiO2 lattice and also results in reduction of the amount of oxygen vacancy, which is in qualitative agreement with our previous result of transient absorption measurement for rutile TiO2:N,F. The role of fluorine in TiO2:N,F is thus clarified through our systematic first-principles calculations.