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Description: Despite the growing threat of pharmaceutical pollution, we lack an understanding of whether and how such pollutants influence animal behaviour in the wild. Using laboratory- and field-based experiments across two years in Atlantic salmon (Salmo salar; n = 698), we demonstrate that the commonly detected anxiolytic pollutant clobazam accumulates in the brain of exposed fish and alters river-to-sea migration success. Clobazam exposure increased the speed with which fish passed through two hydropower dams along their migration route, resulting in greater migration success compared to controls. We found that such effects were associated with altered shoaling behaviour in fish exposed to clobazam. Drug-induced behavioural changes, even when seemingly beneficial, are expected to have wide-ranging consequences for the ecology and evolution of wild populations.