AbstractIntroductionInconsistent toilet usage is a continuing challenge in India. Despite the impact of social expectations on toilet usage, few programs and studies have developed theoretically grounded norms-centric behavior change interventions to increase toilet use in low-income settings. This protocol details the rationale and design of an ex-ante, parallel cluster-randomized trial evaluating the impact of a demand-side, norms-centric behavior change intervention on exclusive toilet use and maintenance in peri-urban Tamil Nadu, India.Methods and AnalysisFollowing two years of formative research, we developed an evidence-based norm-centric behavior change intervention called Nam Nalavazhvu (Tamil for “Our wellbeing”). The multi-level intervention aims to shift collective beliefs by shifting empirical expectations or beliefs about other relevant people’s sanitation practices. It also provides action-oriented information to aid individuals to set goals and overcome barriers to build, consistently use and maintain their toilets. This trial includes 76 wards in Pudukkottai and Karur districts, where half were randomly assigned to receive the intervention and the remaining serve as counterfactuals. During baseline and endline (conducted one year after the initiation of intervention) assessments, we collect relevant data and compare results between study arms to determine the impacts of the Nam Nalavazhvu intervention on sanitation-related behavioral, health, wellbeing outcomes and potential moderators. This study is powered to detect differences in the prevalence of exclusive toilet use between study arms. We will also conduct a process evaluation to understand the extent to which the intervention was implemented, as designed.Ethics and DisseminationThe study protocol has been reviewed and approved by the ethics board at the University of Pennsylvania, USA and the Catalyst Foundation, India. Research findings will be disseminated through open access peer reviewed publications and presentations to stakeholders, government officials and conferences.Trial registrationNCT04269824.Strengths and limitations of this studyThis ex-ante, parallel cluster randomized trial assesses the impact of a norm-centric behavior change intervention strategy to improve sanitation practices. These behavior change techniques are novel to the sanitation sector but has been effective in changing a variety of behaviors, such as water use, drinking behavior, and energy consumption.The study outcomes include health, wellbeing outcomes, and a careful assessment of changes in social beliefs, expectations, and social determinants of collective sanitation behaviors.This study is being rolled out during the ongoing COVID-19 pandemic. This can potentially impact the effectiveness of this intervention package that uses community and network-based group activities. However, through a detailed process monitoring and evaluation we will be able to assess the impact on delivery, and subsequent behavior change in this unique setting.