Handwashing with soap at critical times helps prevent diarrhoeal diseases. Changing handwashing practices through behaviour change communication remains a challenge. This study designed and tested a scalable intervention to promote handwashing with soap. A cluster-randomised, controlled trial compared our intervention against standard practice. Subjects were men, women and children in 14 villages in Cross-River state, Nigeria. The primary outcome was the proportion of observed key events on which hands were washed with soap. Binomial regression analysis calculated prevalence differences between study arms. The intervention had minimal effect on the primary outcome (+2.4%, p = 0.096). The intervention was associated with increased frequency of handwashes without soap before food contact (+13%, p = 0.017). The intervention failed to produce significant changes in handwashing with soap at key times. The low dose delivered (two contact points) may have increased scalability at the cost of effectiveness, particularly in the challenging context of inconvenient water access.