Previous research has suggested that the presence of zoo visitors may be stressful for various primate species, and visual contact with visitors may be an important sensory stimuli involved. We studied a group of black-capped capuchins, Cebus apella, in a controlled experiment, randomly imposing two treatments: customized one-way vision screens on the exhibit viewing windows to reduce visual contact with visitors; and unmodified viewing windows that allow full visual contact with visitors. We sampled capuchin behaviour including intra-group aggression, vigilance and abnormal behaviours. To provide a measure of physiological stress, we also analysed capuchin faecal samples for glucocorticoid metabolite (FGM) concentration. When the view of visitors was obscured, we found marked reductions in capuchin aggression and FGM concentration among all adults, as well as reductions in abnormal behaviour in two individuals. The capuchins also avoided the visitor viewing area in the unmodified viewing window treatment. These results suggest that reducing the capuchins’ ability to view visitors improved their welfare. However we also found a reduction in the number of visitors when the one-way vision screens were in place suggesting that the visitor experience may have been compromised. These results highlight a possible dilemma for the zoo industry between enhancing welfare in primates and providing for visitor experience.