Behavioral economics hold great promise in changing patterns of behavior that influence human health. Handwashing with soap is one such behavior that is important in reducing exposure to pathogens, and in school-age children, handwashing helps reduce absenteeism through the prevention of respiratory and diarrheal diseases. However, the gap between knowledge on the importance of handwashing and actual handwashing practice, especially with soap, persists. Many traditional behavior change communication approaches have failed in achieving and sustaining improved handwashing practices. Cognitive psychology research on habits as well as nudge theory, a component of behavioral economics predicated on the idea of making a behavior as easy as possible to do, suggests that introducing a disruptive cue into the environment may be able to interrupt current habitual neurological patterns to effect and then sustain behavior change. We used a participatory process to identify and introduce a locally appropriate disruptive cue to improve handwashing behavior in schools in Zambia. We then utilized a school-randomized controlled trial to test the soap-on-a-rope in 50 government schools in Namwala District of Southern Province. Two outcomes were considered among school children; washing hands with water and using soap while washing hands. Following the intervention, soap use was more likely in intervention schools than control schools [Odds ratio = 7.23, 95% confidence interval = (1.76–29.71)], though both intervention and control schools saw an increase in handwashing without soap. This low-cost intervention could be scaled throughout Zambia and may work well in other countries of similar circumstances.