BACKGROUND: Secondary education and delayed marriage provide long-term socio-economic and health benefits to adolescent girls. We tested whether a structural and norms-based intervention, which worked with adolescent girls, their families, communities, and secondary schools to address poverty, schooling quality and gender norms, could reduce secondary school drop-out and child marriage among scheduled-caste/scheduled-tribe (SC/ST) adolescent girls in rural settings of southern India. METHODS: 80 of 121 villages in Vijayapura and Bagalkote districts, Karnataka State, were randomly selected (control = 40; intervention = 40). All 12-13 year-old SC/ST girls in final year of primary school (standard 7th) were enrolled and followed for 3 years (2014-2017) until the end of secondary school (standard 10th). Primary trial outcomes were proportion of girls who completed secondary school and were married, by trial end-line (15-16 years). Analyses were intention-to-treat and used individual-level girl data. RESULTS: 92.6% (2275/2457) girls at baseline and 72.8% (1788/2457) at end-line were interviewed. At end-line, one-fourth had not completed secondary school (control = 24.9%; intervention = 25.4%), and one in ten reported being married (control = 9.6%; intervention = 10.1%). These were lower than expected based on district-level data available before the trial, with no difference between these, or other schooling or sexual and reproductive outcomes, by trial arm. There was a small but significant increase in secondary school entry (adjusted odds ratio AOR = 3.58, 95% confidence interval CI = 1.36-9.44) and completion (AOR=1.54, 95%CI = 1.02-2.34) in Vijayapura district. The sensitivity and attrition analyses did not impact the overall result indicating that attrition of girls at end-line was random without much bearing on overall result. CONCLUSIONS: Samata intervention had no overall impact, however, it added value in one of the two implementation districts- increasing secondary school entry and completion. Lower than expected school drop-out and child marriage rates at end-line reflect strong secular changes, likely due to large-scale government initiatives to keep girls in school and delay marriage. Although government programmes may be sufficient to reach most girls in these settings, a substantial proportion of SC/ST girls remain at-risk of early marriage and school drop-out, and require targeted programming. Addressing multiple forms of clustered disadvantage among hardest to reach will be key to ensuring India "leaves no-one behind" and achieves its gender, health and education Sustainable Development Goal aspirations. TRIAL REGISTRATION: ClinicalTrials.gov registration number NCT01996241.