Endocrine-disrupting chemicals are accumulating in environments globally. This includes trenbolone, a potent growth-promoting steroid that enters waterways in agricultural run-off. However, whether and how endocrine disruptors like trenbolone impact complex behaviours in wildlife remain largely unknown. We exposed male guppies (Poecilia reticulata) to trenbolone and compared the response of exposed and unexposed males to sequentially presented large and small females. Due to a positive size-fecundity relationship, larger females are generally expected to be preferred by males. While we found no evidence that the size of a previously encountered female affected the amount of mating behaviour performed by males during the second presentation, males from both exposure treatments conducted more frequent courting events towards larger females during both presentations, suggesting an absolute preference for greater female size. Further, across both presentations, trenbolone exposure caused a shift in male mating strategy towards increased 'sneaking' behaviour, with major potential implications for reproductive dynamics in exposed populations. Taken together, our findings contribute to a growing understanding of the impacts of endocrine disruptors on complex reproductive behaviours in wildlife.