Handwashing is widely considered the most effective method of preventing the spread of infectious illness. Exploring the determinants of handwashing is vital to the development of interventions to increase this behaviour. A survey based on Social Norms Theory assessed handwashing frequency and perceptions of peer handwashing in 255 university students. Participants reported their own handwashing frequency, and how often they thought their peers washed their hands in particular circumstances, to determine whether misperceptions around handwashing exist, and whether these influence the behaviour of individuals. Gender was found to be a significant determinant of handwashing frequency as females reported washing their hands significantly more often than males. Participants also believed they washed their hands significantly more frequently than their peers. Perceived peer handwashing frequency was significantly correlated with participants’ own behaviour. This effect was seen in overall handwashing and in food, waste and illness-related hand washing. These results suggest perceived social norms around hand washing have a clear association with individual behaviour. Future research might test the effectiveness of a social norms intervention in other settings which carry an increased risk of infection spread.