Photon upconversion based on sensitized triplet?triplet annihilation has been observed on nanocrystalline ZrO2 films cosensitized with platinum(II) octaethylporphyrin (triplet sensitizer) and 9,10-diphenylanthracene (singlet emitter) under sunlight-like conditions (noncoherent excitation source, excitation light intensity as low as 5 mW/cm2). Time-resolved emission measurements showed a fast rise of the upconverted signal (≀10 ns), suggesting that triplet energy migration most probably occurs through a ?static? Dexter mechanism. To the best of our knowledge, this is the first observation of photon upconversion based on sensitized triplet?triplet annihilation on a sensitized mesoporous metal oxide. Implementation of similar systems in dye-sensitized solar cells would increase the maximum theoretical efficiency of these devices from 30% to over 40%.