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We present the design and synthesis of a novel optically transparent platform based on the combination of NaYF4: Er3+, Yb3+ upconverting nanoparticles (UCNPs) and mesoporous metal oxide thin films (MOTFs). The UCNP were obtained by the thermolysis method and were deposited onto a glass substrate by spin coating. The MOTFs (TiO2, ZrO2 and SiO2), prepared by the combination of the sol‐gel approach and the self‐assembly of surfactants, were subsequently deposited on top of the particles giving rise to an ultrathin composite. The characterization of the obtained composite films indicates that they blend the properties of both starting materials: they exhibit upconverting properties, as they are capable of emitting visible light when irradiated in the NIR region, and the MOTFs deposited on top provide an ordered array of mesopores with highly accessible porosity. As the UCNPs emission properties depend on their surrounding temperature, the composite films were tested for temperature sensing applications, using both emission intensity and lifetimes of different signals, with promising results. [ABSTRACT FROM AUTHOR]