We present density functional theory calculations on the effect of subsurface titanium interstitial defects on the adsorption of O2 and water on the anatase (101) surface. Our calculations show that for O2 the strength of the adsorption is largely determined by the availability of electronic charge at specific adsorption sites above the interstitial, whereas for water the adsorption is mainly influenced by defect induced surface distortions. In particular, we found that the presence of a shallow subsurface interstitial makes O2 adsorption very favorable, especially at surface 5-fold Ti sites above the defect, where the computed adsorption energy is as large as 2.5 eV. Lower lying interstitials have a less pronounced effect, since the excess electrons from the defect localize further down below the surface. For the case of water, instead, the adsorption energy does not depend significantly on the depth of the interstitial.