We analyzed data from a cluster-randomized controlled trial conducted among 20 schools in Rajshahi, Bangladesh, to explore the role of social influence on handwashing with soap (HWWS) in a primary school setting. Using data collected through covert video cameras outside of school latrines, we used robust Poisson regression analysis to assess the impact of social influence-defined as the presence of another person near the handwashing location-on HWWS after a toileting event. In adjusted analyses, we found a 30% increase in HWWS when someone was present, as compared with when a child was alone (Prevalence ratio 1.30; 95% confidence interval: 1.14-1.47, P < 0.001). The highest prevalence of HWWS was found when both child(ren) and adult(s) were present or when just children were present (64%). Our study supports the conclusion that the presence of another individual after a toileting event can positively impact HWWS in a primary school setting.