Trimipramine (TMP) is among widely used tricyclic antidepressant drugs throughout the world. It could cause toxic effects to humans and have a high risk of serious cardiovascular side effects; therefore it is important to study its environmental behavior. This research focuses on the effectiveness of ultraviolet (UV) radiation on the photodegradation of TMP, investigating the effect of different environmental variables (initial drug concentration, experimental temperature and solution pH) and different aqueous matrices (ultrapure, tap, surface and wastewater) on the photodegradation process as well as on the formed photo-transformation products (PTPs). The degree of mineralization was studied by following the time course of dissolved organic carbon (DOC) during photodegradation process. Liquid chromatography coupled to ion-trap mass spectrometry (LC–MSn) was employed to monitor and identify the formed PTPs. Biodegradation testing of TMP and samples after the photodegradation process was studied using two tests from the OECD series: Closed Bottle test (CBT; OECD 301 D) and Manometric Respirometry test (MRT; OECD 301 F).Photodegradation kinetics followed first order reactions in all experiments except in wastewater matrix, which showed second order kinetics. The rate constants ranged between 0.0004 sec-1 and 0.0013 sec-1. With regard to water type, TMP photodegradation rate decreased in the order: surface > tap > ultrapure > wastewater. The photodegradation process was highly dependent on the mediums’ pH, especially because TMP solubility decreased by increasing pH. Photodegradation rate of TMP was increased by increasing temperature or decreasing the initial drug concentration. Comparison of the time course of TMP photodegradation by HPLC with that of the corresponding DOC values indicates that TMP was transformed to new PTPs without complete mineralization. The proposed phototransformation pathway of TMP in aqueous medium was by hydroxylation mainly. Finally, biodegradation tests revealed that neither TMP nor its PTPs were biodegradable under the studied conditions.Toxicity studies are required to be applied for the further characterization of the environmental and human risks due to the presence TMP and its PTPs in the environment.