Previous research has suggested that the presence of zoo visitors may be stressful for various primate species, and visual contact with visitors may be the sensory stimuli that mediate visitor effects. We studied a group of black-capped capuchins, Cebus apella, in a controlled experiment, randomly imposing two treatments: customised 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 and other social interactions, 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 (from 14.5 bouts to 4.6 bouts per weekend, P = 0.004) and FGM concentration (from 620 to 410 ng/g, P = 0.008) among all adults, as well as reductions in abnormal behaviour (P = 0.01) in two individuals. The capuchins also avoided the visitor viewing area (P = 0.003) 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 visual contact was reduced (from an average of 23 visitors per scan to 15, P = 0.008), suggesting that the visitor experience may have been compromised by the lack of interaction with the capuchins. These results highlight a possible dilemma for the zoo industry between enhancing animal welfare in primates and providing for visitor experience.