Density functional theory simulations are used to investigate the formation and mobility of Ti interstitial ions, Ti_i, at the (110) surface of rutile TiO₂. Interstitials were found to be favoured in the second layer below the surface plane, where they induce electron polaron states at surface and subsurface lattice Ti atoms. Reduction of the surface significantly lowers the barrier for Ti_i formation at the surface: the barrier for formation of Ti_i is reduced to just ∼0.5 eV for a Ti atom next to two bridging oxygen vacancies. However, the barrier to separate the interstitial from the surface oxygen vacancies is ∼2.5 eV. The bulk diffusion barrier is recovered after the interstitial is moved away from the vacancy complex. These results support an experimentally postulated mechanism of Ti_i formation and contribute to our understanding of the TiO₂ surface reduction and reoxidation.