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We report that the valence and conduction band energies of TiO₂ can be tuned over a 4 eV range by varying the local coordination environments of Ti and O. We examine the electronic structure of eight known polymorphs and align their ionization potential and electron affinity relative to an absolute energy reference, using an accurate multiscale quantum-chemical approach. For applications in photocatalysis, we identify the optimal combination of phases to enhance activity in the visible spectrum. The results provide a coherent explanation for a wide range of phenomena, including the performance of TiO₂ as an anode material for Li-ion batteries, allow us to pinpoint hollandite TiO₂ as a new candidate transparent conducting oxide, and serve as a guide to improving the efficiency of photo-electrochemical water splitting through polymorph engineering of TiO₂.