TiO2 and Fe(S)-doped TiO2 photocatalysts have been synthetized by a microwave-assisted method and deeply characterized by means of several techniques (N2 adsorption–desorption isotherms, scanning electron microscopy (SEM), TEM, high resolution transmission electron microscopy (HRTEM), powder X-ray diffraction (XRD), temperature-programmed desorption of NH3 (TPD-NH3) and UV–vis diffuse reflectance, Raman and XPS spectroscopic techniques). The synthetized samples were tested in the photodegradation of methyl red dye under UV irradiation at room temperature. Depending on the temperature and duration of the microwave synthesis, some changes in the textural and crystalline structure of the titanium dioxide were observed which influenced on the photocatalytic activity. The best photocatalyst found in this study was the one doped with S (0.1 wt.%) prepared by microwave-assisted synthesis at 215 °C for 60 min and calcined at 550 °C. The enhancement of color removal over this sample (49% of color removal) was associated to the formation of anatase phase and the enhancement of the sample acidity with respect to undoped TiO2 sample. A volcano-shaped curve was obtained for methyl red degradation against TiO2 crystal size. Catalyst calcination at 700 °C led to a drastic drop in activity due to formation of large TiO2 crystals of rutile phase, as confirmed by XRD.