Wastewater has been one of the major issues concerned by researchers in developing countries such as Indonesia. The wastewater contaminants can be removed by a photocatalytic process using a photocatalyst material, and one famous is Strontium Titanate (SrTiO3 or STO). STO doped with metals, one of which is Manganese (Mn), can improve its photocatalytic activity.Synthesis of Mn-doped STO (SrTi1-xMnxO3, where x=0.05-0.2) has been completed via the coprecipitation method to examine the influences of Mn concentrations on the microstructure, chemical bonds, and photocatalytic activity of STO material. The characterization employed X-Ray Diffraction (XRD), Fourier Transform Infra-Red (FTIR), and UV-Vis spectrophotometer. The data revealed that Mn-doped STO samples had been formed with high crystallinity. The photocatalytic activity of the Mn-doped STO samples was examined with the degradation of 10 ppm methylene blue (MB) under UV irradiation. The yields demonstrated that all samples have successfully become photocatalysts by degrading MB. The degradation was affected by the crystallinity and surface area of the samples. The study found that the best Mn-doped STO resulting in 54.69% MB degradation was from x=0.15 with 5 hours irradiation.