<p>Icaridin is currently one of the most widely used insect repellents. At the same time, it is widely present in the natural environment. In this study its photochemical transformation was explored for the first time. The studied substance turned out to be extraordinarily photostable, especially in the circumneutral environment, which substantially increases probability of its persistence. Basic pH significantly accelerated the transformation process. Presence of the dissolved organic matter (humic and fulvic acids) also resulted in the visible increase of degradation efficiency, however to a lesser extent. This phenomenon was also observed in the waters of natural origin (river and lake). Overall 12 transformation products (TPs) were detected in the studied processes—2 major (aldehyde and 2-ethylpiperidine derivatives) and 10 minor (products of hydroxylation, oxidation and rearrangement). Phototransformation of icaridin resulted in significant increase of toxicity to <i>D. magna</i>. Moreover the toxicity mechanism of the TPs is probably of a different nature compared to the parent compound: the effect was delayed and was accompanied by change of swimming activity of the tested organisms. Those findings indicate that the environmental safety of icaridin should probably be revisited.</p>