The role of the sputtering power, film thickness, and sputtering working pressure on the adsorption capacity and photocatalytic activity of MoO3 thin films was studied. The MoO3 thin films were deposited at room temperature by radio frequency reactive magnetron sputtering and subsequently post-annealed in air at 500 degrees C. X-ray diffractograms revealed the presence of the alpha- and beta-MoO3 crystalline phases. The thin film adsorption was assessed by the removal of methylene blue (MB) from aqueous solutions under dark conditions. In addition, the photocatalytic activity for the most adsorbent films was evaluated by the degradation of MB aqueous solutions under UV-irradiation. Raman spectroscopy measurements showed a strong MB adsorption on the film surface. Photoacoustic spectroscopy experiments confirmed the adsorption of MB on the catalyst surface; specifically, a strong hypsochromic shift of the absorption bands indicated that MB forms aggregates on the MoO3 surface. The maximum adsorption and photocatalytic performances were observed for the samples deposited at 60 W and 12.0 mTorr, with a thickness of 400 nm. The results are explained in terms of the stoichiometry, as well as the structural, optical, and morphological properties of the films.