Pharmaceuticals occur in the environment due to their excessive consumption and the inefficiency of treatment plants to degrade, inactivate or remove them. Diazepam (DZP) stands out as the most consumed benzodiazepine, and induces sedative effects and reduces anxiety. Considering its potential appearance in several environmental compartments, the aim of the present study was to determine the effects of DZP under acute and chronic exposures on the mysid Mysidopsis juniae. Mortality was assessed using an acute toxicity test and a LC50 of 3.7 ± 0.5 mg·L-1was derived. The mass and length of the mysids was recorded in the chronic exposure to test for sublethal effects, and concentrations of 0.25 and 0.5 mg·L-1 of DZP affected mysids length and dry weight, respectively. Although effect-inducing concentrations used in this study are above environmentally relevant levels, the present study adds value to the limited available data for DZP toxicity to marine organisms, and we have shown that M. juniae is the most sensitive marine crustacean species tested thus far.