A first-principles study on the adsorption of 3d transition metal atoms on a stoichiometric TiO2(110) surface is reported. For all 3d elements except Cu, the most stable on-surface adsorption site is a site where the adatom binds to two twofold and one threefold surface oxygen atoms. For Ti, V, and Cr, however, a subsurface site, where the adatom substitutes a sixfold Ti atom, is more stable. The adatoms are oxidized in all cases. The charge transfer to the substrate is larger for the substitutional site than for the on-surface adsorption sites and decreases with atomic number along the 3d series. The relative stabilities of the adsorption sites are discussed in terms of the charge state of the adatoms, the electronegativity of their neighbors, and the metal−oxygen bond enthalpies of the 3d elements. The results indicate that, at submonolayer coverage, the early 3d elements wet the surface and may diffuse into the substrate, whereas the late 3d elements tend to form large three-dimensional clusters.