The effect of manganese doping on the electronic structure of titania in the rutile modification is investigated theoretically. The relative stability of the chemically accessible Mn oxidation states +I to +VII for MnTi defects is calculated in dependence of the spin state. For all MnTi oxidation states except +IV, additional lattice defects such as O and Ti vacancies were introduced to obtain charge neutrality. According to the calculated formation energies, Mn4+ is the most stable oxidation state for MnTi substitution in comparison with Mn3+ and Mn2+ doping. Depending on the Mn oxidation state, occupied and unoccupied states are created in the titania band gap.