Wavelength-dependent refractive index functions (RIFs) of (TiO2)(n), nanoparticles (n = 2, 8, 18, 28, or 38) have been calculated by using the data from our previous density functional theory and time-dependent density functional theory photoabsorption calculations. The results show significant blueshifts and increased anisotropy in the RIFs of the nanoparticles, when compared to experimental bulk values. On the basis of the results, we conclude that, in the case of these 400 ultrasmall particles, the RIFs may depend notably on the shape and structure of the cluster and on the other hand the fundamental absorption characteristics do not depend much on the rather limited cluster size range. The results also support the proposition that, in light-scattering measurements, one should not use the bulk RIF to model nanosize particles, at least in the case of TiO2 particles. Our results shed some light into this computationally and experimentally very challenging area of nanoparticle properties.