Handwashing is a critical hygiene practice in preventing infectious diseases. In Arusha, Tanzania, community-based handwashing promotion programmes have been implemented to improve health knowledge and behaviour. A comprehensive search was conducted for relevant studies, focusing on randomized controlled trials (RCTs) that evaluated the impact of community-based handwashing promotion programmes. Studies were included if they provided data on pre- and post-intervention health knowledge scores among residents. Analysis revealed a statistically significant increase in health knowledge scores related to disease prevention after the intervention, with an effect size estimate (Cohen's d) of 0.56 (95% CI: 0.41–0.72). Community-based handwashing promotion programmes appear effective in enhancing residents' health knowledge about preventing diseases. Further studies should explore the long-term effects and scalability of these interventions to broader populations. Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.