To determine whether Rojiroti microfinance, for poor Indian women, improves child nutrition.Cluster randomised trial.Tolas (village communities) in Bihar State.Women and children under 5 years.With Rojiroti microfinance, women form self-help groups and save their money to provide loans to group members. After 6 months, they receive larger external loans. Tolas were randomised to receive Rojiroti immediately or after 18 months.The primary analysis compared the mean weight for height Z score (WHZ) of children under 5 years in the intervention versus control tolas who attended for weight and height measurement 18 months after randomisation. Secondary outcomes were weight for age Z score (WAZ), height for age Z score, mid-upper arm circumference (MUAC), wasting, underweight and stunting.We randomised 28 tolas to each arm and collected data from 2469 children (1560 mothers) at baseline and 2064 children (1326 mothers) at follow-up. WHZ was calculated for 1718 children at baseline and 1377 (674 intervention and 703 control) at follow-up. At 18 months, mean WHZ was significantly higher for intervention (-1.02) versus controls (-1.37; regression coefficient adjusted for clustering β0.38, 95% CI 0.16 to 0.61, p=0.001). Significantly fewer children were wasted in the intervention group (122, 18%) versus control (200, 29%; OR=0.46, 95% CI 0.28 to 0.74, p=0.002). Mean WAZ was better in the intervention group (-2.13 vs -2.37; β0.27, 95% CI 0.11 to 0.43, p=0.001) as was MUAC (13.6 cm vs 13.4 cm; β0.22, 95% CI 0.03 to 0.40, p=0.02). In an analysis adjusting for baseline nutritional measures (259 intervention children and 300 control), only WAZ and % underweight showed significant differences in favour of the intervention.In marginalised communities in rural India, child nutrition was better in those who received Rojiroti microfinance, compared with controls.NCT01845545.