We employ time-dependent density functional theory to study the optical properties of a squaraine sensitized TiO2 system, as a model for the corresponding dye-sensitized solar cell. The all-organic squaraine dye is particularly promising for light absorption in the red spectral region. We compute the photoabsorption spectrum of a periodic TiO2 slab exposing anatase (101) surfaces, both for the clean slab and for the surface covered by the squaraine dye. Those spectra are compared to recent experimental data. The main absorption peaks are analyzed, concluding that both direct and indirect charge injection mechanisms are active in this system. (c) 2009 Elsevier B.V. All rights reserved.