IntroductionLocal coalitions can advance public health initiatives such as smoke-free air but have not been widely used or well-studied in low-income and middle-income countries.MethodsWe conducted a matched-pairs community-randomised controlled trial in 28 communities in Armenia and Georgia (N=14/country) in which we helped establish local coalitions in 2019 and provided training and technical assistance for coalition activity promoting smoke-free policy development and enforcement (2019–2021). Surveys of ~1450 households (Fall 2018, May–June 2022) were conducted to evaluate coalition impact on smoke-free policy support, smoke-free home adoption, secondhand smoke exposure (SHSe), and coalition awareness and activity exposure, using multivariable mixed modelling.ResultsBivariate analyses indicated that, at follow-up versus baseline, both conditions reported greater smoke-free home rates (53.6% vs 38.5%) and fewer days of SHSe on average (~11 vs ~12 days), and that intervention versus control condition communities reported greater coalition awareness (24.3% vs 12.2%) and activity exposure (71.2% vs 64.5%). Multivariable modelling indicated that intervention (vs control) communities reported greater rates of complete smoke-free homes (adjusted Odds Ratio [aOR] 1.55, 95% confiedence interval [CI] 1.11 to 2.18, p=0.011) and coalition awareness (aOR 2.89, 95% CI 1.44 to 8.05, p=0.043) at follow-up. However, there were no intervention effects on policy support, SHSe or community-based activity exposure.ConclusionsFindings must be considered alongside several sociopolitical factors during the study, including national smoke-free policies implementation (Georgia, 2018; Armenia, 2022), these countries’ participation in an international tobacco legislation initiative, the COVID-19 pandemic and regional/local war). The intervention effect on smoke-free homes is critical, as smoke-free policy implementation provides opportunities to accelerate smoke-free home adoption via local coalitions.Trial registration numberNCT03447912.