BackgroundNiger has the highest rate of adolescent fertility in the world, with early marriage, early childbearing and high gender inequity. This study assesses the impact of Reaching Married Adolescents (RMA), a gender-synchronized social behavioral intervention designed to improve modern contraceptive use and reduce intimate partner violence (IPV) among married adolescent couples in rural Niger.MethodsWe conducted a four-armed cluster-randomized trial in 48 villages across three districts in Dosso region, Niger. Married adolescent girls (ages 13–19) and their husbands were recruited within selected villages. Intervention arms included home visits by gender-matched community health workers (CHWs) (Arm 1), gender-segregated, group discussion sessions (Arm 2), and both approaches (Arm 3). We used multilevel mixed-effects Poisson regression models to assess intervention effects for our primary outcome, current modern contraceptive use, and our secondary outcome, past year IPV.ResultsBaseline and 24-month follow-up data were collected April–June 2016 and April–June 2018. At baseline, 1072 adolescent wives were interviewed (88% participation), with 90% retention at follow-up; 1080 husbands were interviewed (88% participation), with 72% retention at follow-up. Adolescent wives had higher likelihood of modern contraceptive use at follow-up relative to controls in Arm 1 (aIRR 3.65, 95% CI 1.41–8.78) and Arm 3 (aIRR 2.99, 95% CI 1.68–5.32); no Arm 2 effects were observed. Relative to those in the control arm, Arm 2 and Arm 3 participants were significantly less likely to report past year IPV (aIRR 0.40, 95% CI 0.18–0.88 for Arm 2; aIRR 0.46, 95% CI 0.21–1.01 for Arm 3). No Arm 1 effects were observed.ConclusionsThe RMA approach blending home visits by CHWs and gender-segregated group discussion sessions is the optimal format for increasing modern contraceptive use and decreasing IPV among married adolescents in Niger.Trial registrationThis trial is retrospectively registered with ClinicalTrials.gov, Identifier NCT03226730Supplementary InformationThe online version contains supplementary material available at 10.1186/s12978-023-01609-9.