In this study TiO2-mediated photocatalytic degradation of the persistent drug clozapine under the simulated solar radiation was studied for the first time. The experiments were conducted both in the ultrapure and river water, which enabled the assessment of the organic matrix impact. The direct and indirect photolysis experiments were conducted for a comparison. Influence of the catalyst loading on the efficiency of the process was also assessed, and the highest catalyst loading (300 mg L−1) was found to be the most effective. The TiO2 photocatalysis was extremely effective for clozapine degradation – the decomposition was almost 300 times faster in comparison to the direct photolysis (t1/2 = 1.7 min, neither clozapine, nor the intermediates were detected after 20 min of irradiation), and presence of the organic matrix did not negatively affect the process. Nevertheless the photocatalytic process turned out to be highly sensitive to act of the ROS scavengers. Thirteen transformation products (TPs) were found and their structures were elucidated by the means of high resolution mass spectrometry. Properties – toxicity, biodegradability, BCF and BAF – of TPs and the parent molecule were estimated with the use of computational methods. Identified TPs were found as generally less toxic and more biodegradable than clozapine.