The Cholera-Hospital-Based-Intervention-for-7-days (CHoBI7) is a water treatment and handwashing with soap intervention for diarrhoea patients and their household members which is initially delivered in a healthcare facility setting. This study evaluated the effectiveness of CHoBI7 program delivery in increasing handwashing with soap in a healthcare facility setting among diarrhoea patients and their household members. A randomised controlled trial of the CHoBI7 program was conducted among 404 diarrhoea patients and their accompanying household members in healthcare facilities in Dhaka, Bangladesh. The 'Standard Message' Arm received the standard message given in Bangladesh to diarrhoea patients on the use of oral rehydration solution. The 'Health Facility Visit+Soapy Water' Arm received the standard message, the CHoBI7 communication module delivered bedside to the patient; and a soapy water bottle in the healthcare facility. The 'Health Facility Visit+Handwashing Station' Arm received this same intervention plus a small plastic handwashing station. Within 24h of intervention delivery, three-hour structured observation of handwashing practices at stool/vomit- and food-related events (key events) was conducted in healthcare facilities of diarrhoea patients and their accompanying household members. Compared to the Standard Message Arm, there was significantly more handwashing with soap at key events in both the Health Facility Visit + Soapy Water Arm (51% vs. 25 %) (Odds Ratio: 3.02; (95% Confidence Interval (CI): 1.41, 6.45) and the Health Facility Visit+Handwashing Station Arm (58% vs. 25%) OR: 4.12; (95% CI: 1.86, 9.14). These findings demonstrate that delivery of the CHoBI7 communication module and provision of a soapy water bottle to diarrhoea patients and their accompanying household members presents a promising approach to increase handwashing with soap among this high risk population in a healthcare facility setting in Bangladesh.