Animals held in captivity often endure more stress than wild congeners do, which might be harmful for their health and wellbeing. This stress is often expressed by showing stereotypic behaviour. Previous studies showed that visitors can have a negative impact on the stress levels of zoo-housed animals, referred to by the term ‘visitor effect’. The visitor effect is found to be strong in arboreal primate species, such as spider monkeys. The current study examined the effect of visitors on the stress-related behaviour of red-faced spider monkeys (Ateles Paniscus) housed at ARTIS Amsterdam Royal Zoo. Two visitor variables were assessed; number of visitors and sound volume. Additionally, this study examined whether there was a difference in visitor effect in the inside and outside compartment of the enclosure since these exhibits differ strongly in design. Behavioural observations were conducted for 3 weeks daily, during which all behaviour displayed by the spider monkeys was noted, along with the number of visitors present, the level of sound (in decibel) and the location of observation. This study demonstrated that an increase in average level of decibel led to an increase in stereotypic behaviour. Additionally, spider monkeys were found to display more stereotypic behaviour in the outside compartment of their enclosure. However, this study did not find the number of visitors to have an influence on the stress-related behaviour. These results suggest that visitor noise can have a negative effect on the stress levels of red-faced spider monkeys and that this effect is dependent on the exhibit design. This study hopefully provides more insight into the visitor effect by assessing the influence of two important visitor variables. These insights might improve future designs of zoo enclosures, thus enhancing animal welfare. However, future research is needed to further assess the cause of stress in zoo-housed animals.