Abstract. The x-ray absorption near-edge structure (XANES) and first-principles calculations were employed to study the local structure and magnetic origin of Co-doped rutile TiO₂ thin film. It is found that the experimental Co K-edge XANES can be reproduced by the calculated spectrum of substitutional Co (Co_Ti) with O vacancy (V_O), suggesting the coexistence of Co_Ti and V_O in this system. First-principles total energy calculations further reveal that the O vacancy prefers to reside near the Co ion and substantially facilitates the impurity doping. The electronic structure analysis indicates that the O vacancy can mediate the ferromagnetic interaction between the Co_Ti ions and thus is responsible for the observed room-temperature ferromagnetism in the Co-doped TiO_2-δ thin films.