Pharmaceutical contaminants pose a significant environmental problem in the world. New pollutants, such as diclofenac (DCF), are ubiquitous in a variety of watercourses, which can compromise water quality. New contaminants cannot be removed by conventional treatment methods and increasing attention is being paid to photocatalytic processes. Photocatalysis is a process for the decomposition of pollutants under the influence of ultraviolet and visible radiation in the presence of photocatalysts. The first step is the adsorption of these substances on the surface of the photocatalyst and then their decomposition by the action of photogenerated radical species. Within this work, the photocatalytic decomposition of diclofenac (DCF) under simulated solar radiation using greigite (Fe3S4), obtained by synthesis in ethylene glycol (Fe3S4 EG) and a mixture of ethylene glycol and water (Fe3S4 EG: H2O) using sol-gel (SG) and hydrothermal (HT) methods, was observed. Greigite is an interesting photocatalyst candidate because of its small bandgap and magnetism. The sol-gel method did not prove to be successful while the hydrothermal method was successful. DCF conversion was 18.7% (Fe3S4 EG HT), while Fe3S4 EG:H2O has not shown activity for diclofenac removal. The results of FTIR spectroscopy show that Fe3S4 EG after photocatalysis under simulated solar radiation is unchanged and ready for reuse. To further improve the photocatalytic activity, the preparation of Fe3S4-TiO2 composite by hydrothermal and sol-gel methods was investigated. However, the aim of improving activity was not achieved, most likely due to poor Fe3S4-TiO2 interphase contact.